It seems that Imgur, to save some storage space, is reducing the quality of the images posted by the Net surfers. So, if you post a picture in JPEG, it will be degraded a third of the time especially if you use red inside it.
On the one hand this is understandable, Imgur always has more and more users, and it is necessary for them to save some money on the servers.
This can be very annoying for you and the Imgurians may not appreciate the lack of pixels and therefore not vote for your meme.
One solution to reduce the chances to get the quality degraded by Imgur is to post the images in PNG format.