You can either have a random picture or a selected one.
The harder is to define which picture is the first: the 1st uploaded, the 1st by shot date , by the rating, the number of comments, etc...
Does the manual selection is too much for you ?
there any way that is automatic. According to the name? eg 001,002,003, photo: 001 series which would represent the category or subcategory.
This is a valuable request.
You can explain it in details in the bugtracker.