Wikimedia Bugzilla is closed!

At the Commons, we have a lot of images that match the regular 
expression
   ^[[:digit:]]+px-
See, for example,
   http://commons.wikimedia.org/wiki/Category:Images_for_cleanup
Virtually without exception, these are MediaWiki's thumbnails that 
users have accidentally downloaded from some other Wikimedia site, 
and uploaded at the Commons.  When uploading an file whose title 
matches the above expression, we'd like a warning similar to 
the "file with this name exists already" warning, that would say 
something like
  This appears to be a thumbnail that you just downloaded from 
  another Wikimedia site.  Please go back and get the
  full-resolution version to upload here.
With the same "Save file" and "Re-upload" options.

See

http://commons.wikimedia.org/wiki/Commons:Village_pump#Disallow_image
s_that_start_with_600px-

for a discussion.