#Method 1
Use a Image Optimization Tool and try again!
http://luci.criosweb.ro/riot/
#Method 2
Try to change the extension.
to .jpg (no transparency)
to .png (with transparency)
to .gif (transparency supported)
Those 3 are recommended!
#Method 3
Convert the image
Google is your friend <3
#Method 4
Redownload the image (may the image caused corruption)
~Molestia