So, turns out it’s a size limit per post - basically each post (including images) can only be ~10mb.
So if you have more images add them with another post in the thread.
Also, a message about the file size limit should now be visible on the upload dialog to make that clear.
POW! 
Thanks for spotting that one!
Thanks for looking into it! I have a follow-up question though, do I need to be worried to be blocked for spamming if I post a bunch of posts with just two images? If so, how could I avoid that?
It shouldn’t be a problem. Try uploading as many as you can, as the system will just reject any that take the post over the 10mb limit.
The system knows you’re not a new user (Trust Level 0) now, so you shouldn’t be targeted as heavily for spamming.
And if you do, I’ll get notified and will unblock you. Probably 
If not, I’ll know and unblock you.
We got this.
It’s super weird, now I can actually upload 4 images per post (so about 16MB), didn’t get a notification and all, but I can only upload the next image for that post when the latest is loaded in the post preview. If I try that before, I get stuck at 1% again.
Oh.
Ok, well, maybe we don’t got this. 
At least it’s working. If you can keep an eye on it and let me know if you get any more info - otherwise I’ll keep my eyes peeled for a fix.
Thanks!
I have a bunch more miniatures for which I plan to upload photos, so there should be plenty of “research material” for this problem 
While the bot’s message kept indicating that no image was included in my replies, the actual images showed up just fine, so perhaps it’s merely a question of that particular step in the bot’s script having an error that causes it to not recognise the image properly? I tried replying with both a .gif and .jpg
If it turns out that it’s just the bot’s introduction reply chain that’s the problem, then it’s quite a minor issue. It was an enjoyable tutorial-conversation with the bot regardless. 
And thanks for the badge, happy to assist, your forum has such a lovely and positive vibe. 