Personally, after seeing the proposed solutions, I would opt to just use
<extptr>. Especially in a case where presentation is the final result and
you are probably going to have a mixture of <dao> or <extptr> depending on
whether the items displayed are from the collection of materials being
described. Looking over <dao> in the EADTL, it states that it is to be
used "...to connect the finding aid information to electronic
representaions of the described materials". This seems to imply that the
<dao> will somehow be a representation of the textual context in which it
occurs, which may or may not be the case.
The images may be part of the collection described, but they may not
necessarily be related to the *information* being conveyed. The images
may be for presentation purposes only, so tagging them as <dao> vs.
<extptr> may not be as important if you don't need (or desire) to maintain
a relationship between the DAO and the textual context in which it occurs.
I suspect that this was one of the main purposes for creating the
distinction between DAO vs. other links in the first place.
Interesting discussion. Thanks for your comments.