Resized images should have individual (detaled) filenames (not just medium.jpg or thumbnail.jpg)

[Aleksej]

Assuming that people want to download the smaller versions of
images, those versions need useful file names.

Currently the names are just "medium.jpg" and "thumbnail.jpg",
which are not unique.

Better names could be something like
"Original\_file\_name.medium.jpg". However, it attention needs to
be paid so that GMG does not try to create files with paths too
long for the operating system to process.