The forum software will always resize it for display, but if you also add it as a link like this:
[img]http://img94.imageshack.us/img94/8962/fenders.jpg[/img]
http://img94.imageshack.us/img94/8962/fenders.jpg
You get this:
http://img94.imageshack.us/img94/8962/fenders.jpg
and clicking on the link will open it in a new window at it's full size. ?
You can also add in the text for the link like this:
[url=http://img94.imageshack.us/img94/8962/fenders.jpg]Click this link for a larger photo[/url]
..and you get this:
Click this link for a larger photo
and if you want to style the text of the link you can too, by surrounding the text with the relevant tags, much as you would in a post. For instance adding the bold tags :
[url=http://img94.imageshack.us/img94/8962/fenders.jpg][b]Click this link for a larger photo[/b][/url]
Gives you this:
Click this link for a larger photo
Hope that helps...