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.