I ended up getting a stock image of a chocolate bunny for the tiny taskbar image since it kept rejecting mine.
FUN FACT: The purple Inzo image is actually a zoomed-in cat's butthole.
The process to upload builds of one's game is hilariously opaque to the point of where I have to assume it's a shitware filter. It did not help that at the command line version of Steam, I was logged into the wrong account, not that it was ever going to tell me. Ah well.