I think when you attach them the way you did the site here limits their size in order to conserve space. To get them bigger, you need to host them on your own space, or someplace like photobucket, and then link them into the thread using the "Insert Image" option. That will maintain them able to open up to original full size.
I just upload them here to the gallery before I make a post. Then you can link to the gallery picture (they give you the link information). It will be small in the post but you can click on the picture and see the larger version in the gallery.