• Scales the image down so that the longest side is 604 pixels if it was bigger than that to start with.
• Recompress all images as JPEGs at quality 85. I found this out with ImageMagick: identify.exe –verbose filenamehere.jpg. Hat tip to Arjan van Bentem for pointing me to IM. It doesn’t look like 85 is all that bad, by the way, if Wikipedia’s sample JPG compressed photos are anything to go by, but do bear in mind that multiple JPEG resaves will degrade the image quality, since it is a lossy format (but of course just copying or downloading it won’t mess with it at all). Also, Facebook will recompress it even if you upload a JPEG image at the correct size (using quality 85 doesn’t work either; it gets recompressed at 85 again)
• Converts image to Grayscale mode if it is a grayscale image. I suspect this might make it smaller.

So, beware of decreased image quality if you are uploading a twice-(or more)-saved JPEG. That being said, I wouldn’t worry about it much, since it’s only Facebook. If you have a slow link to the net, I’d say go right ahead. I wrote about sizing your photos down for faster Facebook uploads a while back, so if you need some instructions, here you are.

Oh, and one more thing. I have found that JPEG compression can be used to artistic effect. Orange Peal Design’s site is a good example (in fact, the only one I know about). Gives it a certain feeling and not just a crappy impression. Strange how these things work.