Not sure if this is unique to me or not, but yesterday when I was finalizing an auction, the game wouldn't let me upload my image until I dropped the resolution to something like 100kb, not the 500kb it's supposed to allow for. It didn't seem to matter whether the file type was gif, jpg, or png, either.

It's definitely not unique to you. When I was making graphics, I'd send some cropped version in order to complete the auction, and then send the player a link to the image, which I always host on my dA account. Most players have some default graphic that they post to the auction to complete it, and then send the link as I did. Either way works, but I never bothered to ask why the game didn't function as it says it should in this case xD

Yup, it’s a problem! One programmer told me it’s because of slight differences in sizing when saving on your computer versus uploading on the web. Because it does work as intended in some cases. However, I’ve asked almost every programmer we’ve ever had if they can fix it and they all haven’t been able to. So finishing auctions like Tana said works! We should probably come up with just a totally different way to do it so it’s not an issue.

I always upload an image with text. It says: Thank you for your order - please check your messages for the finished image. 

Glad it wasn't just me, I was sure I'd done something wrong but then I tried it a bunch of ways and figured it had to be a code issue, lol. It works out okay because I host on imgur and send the final image from there, but that's a great idea about using a default graphic, I'm gonna go ahead and start doing that I think. Way less hassle than resizing an image half a dozen times haha :D