As some of you will have noticed the site went offline altogether over the weekend. Our administrator had to put a lot of work into getting it back in action, but succeeded. However, as John has found there is a problem with photos. I've been experimenting myself, and it seems that Hilary is right - the problem is to do with size. I managed to post any photo successfully so long as the file was under 130kb in size (including multiple photos, so long as each individually was under that size), but failed with any photo over that size.
I have alerted the administrator to this, but in the meantime if you do want to post photos please try to keep them to the very smallest file size you can manage, and definitely under 130kb. If you try but get an error message, try again deleting the first attempt and using a smaller file size.
I'm very sorry about the difficulties, and hope we can sort them out as soon as possible.