what are the hard limitations for uploading images

need to know the exact max limitations since trying to upload isn't working right now.
I did a test with a resize of a lot smaller resolution but keeping it's PPI and those were working fine to initially upload (without publishing)

So it's not a PPI issue which means it's either a resolution issue, a file size issue, or maybe both are in play perhaps.

Knowing what the limitations are will allow me to try and upload the maximum possible quality I can for manuals thanks.

Also, need to be able to bulk add image files. Would make things a lot easier.