The shading looks like it's part of your input file. Especially since it seems to follow the outline of the image contents. You should use an image editor to look at the file you are importing and remove the offending background. Probably an artifact of the image file being created from a scanning operation. Paper is never exactly white.
If it was me I would probably edit the image and set the background to transparent, then write it to a file format that supports transparency, like .PNG. If you're averse to and insist on a .BMP file, you could make sure to fill the background with pure white.
