Click the "Go Advanced" box located next to the "Post Quick Reply" box, after you click it you'll see a box that says "manage attachments" click on it and then you can choose which pictures you want to show. Hope this helps.
if the file type isn't a supported picture file, you'd have to open the picture in windows paint and resave the file with a supported file extension, .jpg, .bmp etc. What does the file end in?