Comment 4 for bug 425874

@mpt:
no, the placeholder image doesn't come with a 404 response code.

A not so clean workaround would be to check the filesize and the last
modified date of the image. If you looked at both of them you could be
reasonably sure if the image returned is or isn't the placeholder image. The
only problems with that would be that once in however many thousand times
you may block an image which you shouldn't and that you would need to roll
out a new update of software-store if the screenshots.debian.org people
modify the image.