WIth Coppermine Picture Gallery, how do I set up a single upload page
for guests so they could in one page upload a single file, with the
bare minimum of surrounding text and graphics? As it's currently
configured, the upload page has lots of text and graphics, and after
you upload, it then asks you to put it into a gallery. I only have one
gallery and I want all pictures that guests upload to go into there,
but I want it to be very easy to upload pictures as a guest, ideally
just a text box for the file name, a browse button, and an upload
button, and that's it. No choosing galleries, albums, categories, etc.
I don't need a title or username or comments or anything else. I've
looked through all the forums but nothing is spot on.

This is not a simple thing to do.  Coppermine, presently, does not
have that feature and it would require a number of custom edits to the
php code to accomplish what you are looking for.