I'm presuming you are wanting something like the Tag option in Facebook, where a pointer and/or box will appear around the specific person. As far as I know, there's no "function" on WikiTree that does that. The only way I can think of doing it would be to actually edit the photo, and have one copy per person with different people highlighted. The other option could be to indicate in the description which one he/she is (second row, fourth from the left), or you could do a large picture and a zoomed in picture, where you select just him/her and a few around them for reference, and make it clear that the one in the center of the zoomed in photo is him/her.