You need to watch the size of the files and the file types...and some files cannot be uploaded for other reasons (maybe the site gets cranky on the server). If you can find other sources in search, try different file types and sizes (resolutions).
You will often be told the parameters to follow with uploading when you go to do it. If you right click (IE) or similar in other browsers and choose 'Properties' ib that menu a window will appear telling you the file name, type, URL/source, size etc and this is what you need to know.
The URL can be pasted in when you click on the 'tree?' icon third from the right (next to remove link) or you can save the file and post it or other files on your computer drive.
There is another routine for photos with the drag down gallery that I think you are frustrated with and there you have tighter restrictions.
Perhaps we ought to have a sticky on the subject?