Issues uploading images onto documents

This is happening on both the desktop and web version of Adobe express on my Macbook Pro - often when I try to upload something to a document from my computer, or sometimes if I try to use images in adobe itself, the image won't appear on the document - just an outline of it with a red triangle with exclamation mark in it. I've tried signing out, uninstalling, switching between desktop and web etc. and it doesn't help. There also doesn't seem to be any logic to it - it may let me upload or use a couple of images but then decides I've had enough and won't allow any others. It also doesn't seem to make a difference what format the images are in. Anyone else had this problem? It's frustrating when you're paying for a service you can't use half the time.