I understand from the upload page that a.net uses image compression of some sort: "Do not worry about long download times for low compression files, our scripts will automatically optimize compression without loss in picture quality.". Indeed some of my images do appear to have smaller file sizes (to be honest I rarely check the size, as I've never had a problem with the 1024kb limit) sometimes 2/3 smaller.
To my eyes, the quality is still there in comparison to the original upload, so how do they do it? Is it just bog-standard jpeg compression? Have any contributors ever noticed quality problems due to it? Just curious.