I think there is a pixel size limit that you're hitting and we can configure that. Newer smartphones and cameras are saving larger and larger sizes, so we'll have to see what size is acceptable for our server and research if it will affect our performance.
I ran into this issue when trying to add a couple of photos to my LOML Yard Sale thread. Yesterday, I had added a 1200x1200, 311KB file and the server resized it to 800x800, 84KB. Today, I tried to add two photos, both 1200x800, one being 207KB, the other 189KB. I got errors on both.
I just tried to upload one of the new photos again as a test. I got the error message when I attempted the 1200x800, 207KB file. When I chose the smaller 800x533, 106KB file I get the following rather than a thumbnail.
When I use the 'Attachments' icon that brings up the images I've uploaded, there are three with a question mark. Is there a way to delete those? They don't show in my USER CP Attachments list.