The help board has some very helpful stickied threads:
https://www.quiltingboard.com/qb-help-center-f27/
It sounds like you are almost there especially if you've found the upload button.
For me and my system, the problem is I'm using an old school digital camera and the file size of the pictures are too large. The 5mb size quoted seams to be for all possible attachments, and the magic number is more like 1kb.
My camera/software doesn't have options so I go to this site (no affiliation) to reduce my file size:
https://www.reduceimages.com/
I simply select their defaults click to reduce, click to download from there, then add to my post here.