As it is currently, using the mediawiki search function, there appears to be no direct image search. Any search that returns images is text-based. To find a picture of a banana, you have to search for "banana" and won't find banana1.jpg. So good luck with that. Maybe if captioning has been done "accurately" throughout, ie according to your needs, or if you are inspired to use the right search term ...
There is one place where you can browse images visually, and that is: "title=Special:ListFiles". This Special Page shows all images as thumbnails along with some info. It can be optionally sorted by date, file size and name but none of these sorts are particularly useful. Marginally more useful is the ability to limit the list by specifying the uploading user. But if i don't know who uploaded the image i'm trying to find, i'm out of luck. Even more so if i want to compare a bunch of images.
Another option that could be easily introduced in this Special Page is Category. All image files, in fact all media files, already know and display which pages link to each file. It should be a relatively easy matter to configure this ListFiles page to restrict results to a specified category, as the wiki will also know which categories those linking pages belong to. Then, only thumbnails in the category i am interested in will be displayed and they will be RELEVANT to my inquiry. This can reduce what i have to plough through by 90% or more, no small triumph in a big wiki.
Several comments below were made before this description was edited.