The one problem with having that iframe thingy in the TiddlyWiki (hey, a rhyme!) : if you click on any link to a picture by mistake, the current web page (i.e. your TiddlyWiki) will navigate to the image.
It might make more sense to have to browser windows open side-by-side. One browser window with the TiddlyWiki, the other browser window to show the folders and files of family photos, and copy link for image from the one browser, and paste into the browser window with the TiddlyWiki.
Ideally, in the TiddlyWiki, you could press a button (Add photo link?), which would create an image tiddler and present a simple form with an edit text widget for setting the _canonical_uri field of the new image tiddler. Just paste the copied image URL into that field, and you now have a nice tiddler (with a nice title), maybe even some tags, to view the image right in the TiddlyWiki.