Are you not able to attach any files at all?
You should be able to attach pictures to messages: when you're on the new post/new reply page, down the page a bit under "Additional Options", you should see a button labelled "Manage Attachments". Just hit that and a pop-up dialogue window will come up that will allow you to attach a file.
If you have a pop-up blocker active, it might be preventing the dialogue window from opening.
Now... our size limit for attached pictures is pretty small, so if you have a decent-sized photo you want to share, it's usually best to host it on a photo upload site like flickr or photobucket and post the link here rather than doing it as an attachment.