To reduce bandwidth the images hosting sites size down the images sometimes, or even they reduce the image quality so when the image reached a certain number of downloads it is showing pixelated due the low quality.
A good site to upload files and link here for the public view is
http://www.imageshack.us/ , under my experience they don't cut the image quality and you can select a no resize option.
Also attaching the good screenshots to the post it will be uploaded and saved in the forum and available for members.