HTTP error when uploading PNG image

I try to upload one of my PNG images to subsite.
If I resize this image below 500x500 px then it works ok. I try with other large JPG files and they work ok. Not sure if this is related to KB/MB size or images sizes as width and height.
We try many things during the chat but none of this work.
I try to contact with the host but they also cannot track issue.