You can't attach a picture if you use "Quick Reply." That is meant just for verbiage. But if you create a "New Thread" or use "Quote" or "Post Reply" You just click on the paper clip icon on the top of the post dialog box. Browse for the photo(s) select then upload the picture(s). Thumbnail(s) appears within the post and you click to enlarge.
If you use quick reply and you decide you would like to add a photo (or do other things not allowed in quick reply) you can click the "go advanced" button below the post dialog box and it will give you full functionality. You can see the "Attached Thumbnail" of a picture I attached below.
Many other forums I use including non-automotive forums, work in a similar or same way.