Hello there, @JMarsh.

We recommend uploading your logo in a square shape. Though you can try uploading a logo in another shape, like rectangle, you may get an undesirable result. The logo might get uploaded but look distorted in QuickBooks Online, or your logo won't really get accepted and uploaded.

I've got a workaround that you may consider. You can crop the rectangular logo to a square to upload it without encountering an error.

Fill me in if you have any other questions about the logo. I'll be around if you need any help.