If you use Internet Explorer, you might want to turn off Smart image dithering
Open Internet Explorer.
Go to Tools menu, select Internet Options. Click on the Advanced tab. Scroll down to the Multimedia section. You will see a checkbox that says "smart image dithering." Uncheck the box if it's checked. Then click on apply.
If it still doesn't work, go to the same place and try unchecking the Enable Automatic Image Resizing box and see if that works.