I've been through the sticky and it doesn't resolve this.
To date, I have been able to upload and add images without problem.
Now, suddenly, I can't.
When I upload, I simply get a white space appearing where the thumbnail should be. There is no sign of the image uploaded to files, yet in media library there is a reference, with a blank thumbnail.
Old posts that have images look fine on the page and I can see the images in uploads, but I cannot see them through media library in the WP dashboard.
The only possible difference between now and the time that image uploads worked is that I installed a ShareThis plugin. I deactivated it, but it didn't change things. Could it have wiped something out?