HD Background Photo Always Blurry

Same thing happened to me.

I also noticed my Pictures used in the screen saver where also low quality now with Mountain Lion.

It's picking my Pictures from Aperture Library.

I believe I've found the answer to this. I've been having the same problem with a picture of my dog that I use in the background, and the usually sharp image looked a little bit blurry. The solution for me was to resize and crop the picture to exactly the resolution of my desktop.

The problem is that the aspect ratio of your image (3:2 I assume) doesn't match the ratio of the screen (16:9 / 4:3). It appears that mountain lion automatically resizes the background image so that the widest dimension fits on the screen (i.e. for the "Fit to Screen" setting). If you choose the "Fill Screen" option, instead of resizing the image from the original, it takes the already downsized version it made to fit the screen and blows it up until it fills the screen, making the image appear blurry.

In my case, I'm using an older Dell monitor with a resolution of 1280x1024. I simply resized/cropped the picture to that resolution, and the image showed up as clear and sharp as ever. It's an annoying work around, especially if you like to change up the background alot, but it seems to work.

Hope that helps.

Thanks for the tip but as you say, you wouldn't want to do this for a 40.000 photo library in Aperture.

I still don't understand why it changed from Lion to Mountain Lion and how come it's not been experienced and tested during beta tests.

I feel I'm not the only one using my Aperture Library for my Desktop pictures.

+1 Same problem. Pics of my desktops here: https://discussions.apple.com/thread/4296648