clearly you can tell I haven't, but I can't see a step that I haven't done.
Let me quote from the docs, highlighting the stuff that you apparently failed to read:
Asking for support on upload issues
When asking for support on the coppermine forum, post a link to your site and a test user account (the test user mustn't be in the admin group!) with upload privileges, with the above mentioned settings in place - this way, supporters can see the error messages as well. Do not post debug_output unless requested. If you want fast results, you should disable admin approval for the group the test user is in, so supporters can tell instantly what is wrong without needing to double-check.
When people have issues with uploading and decide to post their question on the Coppermine support board, they usually are told to read this upload troubleshooting section. Many of them fail to do so properly, which results in frustration both for users as well as supporters. To make this absolutely clear: the above mentioned steps are absolutely mandatory, no matter what skill level you have, no matter what upload method you have troubles with. Failing to do exactly as suggested will result in your request for help being ignored. Yes, this applies to you. We mean it!
And is there any specific reason for not answering the other question:
Does this happen with known-good images as well?
I have asked this for a particular reasons. Just ignoring everything I say will make it more likely that you're going to get ignored as well. After all, I'm not asking for free support from you, but vice versa, so I can expect that you do your homeworks.