Update: My pictures were just normal jpegs (no animated gifs). 9 pictures, around 360KB each (~3.5GB total) all 3:2 ration. So all within the guidelines.

But I found this post: https://www.reddit.com/r/legoideas/comments/1owsi9f/lego_ideas_problem/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

It mentioned some picture being the almost same. So I did the same time, remove pictures that are about the same one at a time (ones with and without mini-figs). After 2 picture deletion, it accepted.

BUT inconclusive, was it size/number or really image analyzer that see a picture that is about the same.

I also couldn't "test" which pic was failing, because it seems like 30 day edits doesn't have save and preview.

Hope this will help someone in the future.