"invalid contest entry"?

size matters
Q: What kind of image files can I enter?
A: In most contests images must be a JPEG or a GIF file. The size of the file must be under 125kb in actual size. JPEG image dimensions must be less than 800 pixels wide by 600 pixels high. Animated GIFs must be less than 550 wide by 450 high. If you are getting an error message, try reducing the file size and/or dimensions just a bit and then uploading again. Upgrade Your Account to a PSC Advantage to break free of these limitations!

I've taken the size and resolution down to ridiculous measures, but continue to get errors hinting at too large a file size.

You have been posting the source image. Not a manipulated one. The point of these contests is to use whatever photo manipulation software you have to create a unique entry. Not change the title, and upload the source.

Well there may have been human error on my part, but the change was subtle but changed the context quite much. Maybe someone missed the manipulation? The picture has been altered again since deleted.