You need to have text in both the Subject box and the Message box. Other than that, have you checked out either the How Do I Insert Photos into a Post
and/or the Using the Insert Inline Image Feature
Another issue you may be having is a timeout because the photos you are attempting to attach are too large and the forum software takes additional time to reduce those attachments. If this is the case, you may be better off to create a new folder on your computer to store photos that you want to attach here, and resize your photos as you move them from their original folder to this new folder. This way, when you attach your photos from this folder the forum software will not need to spend the time to resize you photos and you should not have the timeout problem.