After 2 days of tests, I've discovered that Suffusion 4.4.6 is incompatible with WordPress Download Manager 2.3.7
Every image I download using this plugin is corrupted.
Try to download an image and upload it to Facebook. Or just try to see the image using windows built in image viewers.
I disabled all the plugins first.
After that, I changed the Suffusion theme with Twenty Twelve everything worked fine, no corrupted downloads.
I also started a thread here, because first of all, I thought that the plugin is the problem:
Any help would be greatly appreciated!