I made a little banner for my profile pic, which was 60x10 pixels. I used it, and it was heavily antialiased, looking really bad. I personally think this has to be a BBS bug, because the main product, PICO-8, is a pixel art game. I'd hope to see this fixed at some point

EDIT: my computer might have antialiased it on its own

EDIT EDIT: actually it didn't...

banner images can be any kind of picture, not necessarily pixel art that needs the crisp/pixelated image rendering setting. you can fix it on your end: use a pixel art editor to save your image magnified so that it fits the banner size