You could use,
Code:
width="75%" height="auto"
which shows same results as your edit on my test board.
The only issue I see having with any of the edits is the size of the image.
Example random banner image from the net:
http://shipritestore.com/wp-content/...ns-Banners.jpg
This image is 1664 x 312 and 1941264 bytes (HUGE)
Using style="max-height: 350px; max-width: 75% or width="75%" height="auto will drop the image to 1165 x 218 (Fluid Style)but it will still remain 1941264 bytes (HUGE)
I could add image guidelines & add image resizer link on the profile field page so members will know what size their image needs to be.