New members ned 5 posts before they can post without filling in the capture and questions, just been through that myself, so maybe that's also the threshold for images.
I now have 27 posts and appear to be able to attach an image, although it doesn't show up when I hit 'preview' we shall see if comes out when I hit 'Post' in a moment....
Allowed file types: txt, jpg, gif, pdf, PNG, JPG, img, jpeg, stl
Restrictions: 10 per post, maximum total size 10000KB, maximum individual size 5000KB