I just "upgraded" from snow leopard to mountain lion. I had a custom background image for nearly every single folder. The problem is that 90% of those images are 300dpi, so in Mountain Lion they appear as tiny images that take up 1/4th of the background of the various folders. All of the images are in the Desktop Pictures folder. I thought I could easily fix them by opening them up in preview and dropping the image dpi down to 72dpi. The problem is that preview tells me every image in that folder is locked and cannot be modified and saved. I tried changing the permissions for the folder as a whole and the individual images and it had no effect. Any suggestions?