In Admin Control Panel/General you need to select 'Attachment Settings' and then you'll see an option to adjust 'Total Attachment Quota'. It's likely that the currently set quota has been reached and you simply need to increase it.
Strangely, I was able to copy your caravan picture and then load it back on the board, but not others. ???
Yes, you would because you're simply uploading a pic that's already stored so it just overwrites the old one and doesn't increase file size.