The specification of some Exif tags defined as data type "ASCII" is supporting only ASCII characters. This works well for tags holding e.g. DateTime values but causes problems for tags holding free-text values as most non-English languages use characters beyond the ASCII character set.
In 2008 the Metadata Working Group ([MetadataWG]) issued Guidelines including the advice^*) to use UTF-8 as character encoding for such tags when related XMP properties are updated and the tags are synchronized. Since that time this advice has been widely implemented by software used to edit metadata.
This document adopts this advice and clarifies: only ASCII characters or Unicode characters using UTF-8 encoding should be used for the Exif tags Artist, Copyright and ImageDescription.

*) It is understood that using UTF-8-encoded characters in Exif is formally a violation of the Exif specification permitting only 7-bit ASCII for tags of type ASCII. Therefore some devices (cameras and printers) using the Exif specifications strictly are not able to display non-ASCII characters.

The Exif specification document notes that the Copyright tag can be used to cover the copyright of the photographer of the image as well as the copyright of an editor of the image.
The use for a copyright for the editor is quite rare; but if you do need to include it be aware that the Exif specification directs you to terminate the string with the photographer copyright with a NULL byte and to concatenate the copyright for the editor following. An alternate solution that does not require a NULL byte is to prefix the copyright claim of the photographer with the string Photographer: and the claim of the editor with the string Editor:.
Be aware that all Exif tag values of type ASCII must have a NULL-byte at the end, read the details in the Exif specification.
Be aware that Exif defines ASCII as the type of this tag, read more about the use of ASCII and UTF-8 above.

The Exif specification document defines the Artist tag as recording "the name of the camera owner, photographer or image creator". For mapping purposes a/ the use of only a single name and b/ the use of only the photographer’s or image creator’s (of e.g. a composite photo) name is recommended.
Be aware that Exif defines ASCII as the type of this tag, read more about the use of ASCII and UTF-8 above.

The IPTC property Image Creator uses a structure with the sub-property ImageCreatorName. A mapping of the Exif tag Artist (315,0x13B) to this sub-property may be applied. Read about selecting the photographer’s name from the content of the Artist Tag in the Note on Creator above.

Be aware that the Exif Tags, the XMP property and the IIM properties use different structures and different formats for the values.

The widely used Exif Tag for the description of what an image shows is the Exif ImageDescription. Exif defines ASCII as the type of this tag, read more about the use of ASCII and UTF-8 above.

The Exif Tag is only set by a camera at the time of capture. The value should only be transferred from the Exif Tag to the IPTC property and not from IPTC to Exif.
The Exif Tag ImageUniqueID is defined as a "unique identifier" of an image and its value is a 128-bit (= 16 bytes) numeric value expressed in the hexadecimal format. This makes a string of 32 characters. If this value is created in a way that guarantees a globally unique value it can be used for the IPTC Digital Image GUID property.

If a camera has a built-in GPS receiver or has such a receiver attached one can expect that these tags are set via the GPS data received and this accurately represents the location where the image was created - if so, this perfectly fits a mapping to IPTC’s Location Created.
However, as the values of the GPS Tags can also be set by a geo-location service: where, for example, a photo editor defines a location by names and the service returns GPS data which may be stored in the Exif GPS tags. In this case, the GPS data may not be a good fit for a mapping to Location Created as the used location names refer to a location shown in the image at a distance from the location of the camera. (In such a case the GPS data of such a location does not fit the specification of the Exif GPS tags).

Exif provides three tags for GPS data about a location: GPSLatitude, GPSLongitude and GPSAltitude. But in most cases - see note above - the values of these tags represent the location of the camera which may be at a considerable distance from the location shown in the image.
Therefore IPTC recommends that you do not map these GPS data to Location Shown.

The Exif tag SubjectArea and the IPTC Image Region property have a wide overlapping use. The IPTC Photo Metadata User Guide has a section describing how a mapping between both structured properties can be implemented.

The mapping tables show in the Schema.org Property row the name and identifier of a property. As such metadata properties require to be a property of an object this raises the question: of which object defined by Schema.org?
For the use with an image the answer is simple: it must be the Image Object. The web page about this object shows all properties defined for it and all the properties listed in the mapping tables can be found there.

Be aware that the object Image Object is derived from the superior object Creative Work and inherits all its properties.

Schema.org defines for superior object Creative Work the property about providing a relationship to "the subject matter of the content". This is the same kind of relationship as from an image to a CV-Term About the Image. The value of about can be any Schema.org Thing, this includes any entity or concept defined by a Controlled Vocabulary(CV)-Term structure of the IPTC standard.

IPTC specifies this property as "Globally unique identifier for this digital image". Schema.org has two properties for identifying an Image Object:

Consider for mapping from IPTC to Schema.org:

Schema.org defines for the superior object Creative Work the property genre. It takes either a text of a URL.

Consider for mapping from IPTC to Schema.org these alternative options - be aware that IPTC’s Genre uses a CV-Term as structure for the value:

IPTC’s Web Statement of Rights is defined as "URL referencing a web resource providing a statement of the copyright ownership and usage rights of the image." If the web resource includes usage rights and as they are very close to the terms expressed by a license IPTC suggests to map the URL of this property to Schema.org’s license property of an Image Object having as specification "A license document that applies to this content".