You probably got it Nick. PHP the language of the site only can handle 7 mb of data at a time. This means large pictures can be a problem and as you note there is no helpful error message it just does not work. I normally add large photos one or two at a time by editing the same post, i.e. upload a photo, then edit the same post and upload another photo etc.
There is a bulk upload feature in the Gallery which allows for up to 10 photos at once which is a great time saver until you start running into this issue.
Mark
