I had wanted to insert a big photo into one of my blogs and resize it down in the post and make it clickable to the original size photo. Every time I uploaded it the media library crunched it down to the width of the post in my theme.
I got around this by using Windows Live Writer to do the post and link it to the original in the original size. Now it is sized in the post and clickable to the larger image.
You might want to give that a try.