I understand that you need to protect against spam. I'm a moderator on a writing forum where we have big problems with Spam. But could you have another way of posting pictures that doesn't need a link, or else introduce posting links after 5 posts. I've found myself posting pointless posts just so I can show a picture to get some help.
Don't think outside the box. Think there is no box
You should have been able to just paste the link without using the url tags.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.