I too had trouble attaching pictures when I first started posting.
First, make sure the image you're attaching is under 66560 bytes and is one of the following formats: gif jpg png txt zip bmp jpeg tif.
Second, attach the image file by hitting the Browse button and opening the file you want to attach.
I think the problem is that you don't see the image during preview. Don't worry. If it meets the first criteria it'll be displayed when you post. Try it again.
95 E320 Cabriolet, 131K