I recently updated my blog's header image to a crisp, minimalist banner on a white background, the same one I use for my Etsy shop. I used the same .JPG file for each, but the one in my blogger header appeared to have a grey background while the one for my Etsy shop was white as I liked it.
After searching the Blogger forums I found the solution, and it's easy. Just open your image in whatever editor you use (I used Photoshop Elements), and save your .JPG as a .PNG file instead. Your crisp white background will appear just as you want it.