iandix, the interlift website is wonderful. It has some gorgeous photos.
Great suggestions, Jack! I have been saving my wallpapers as landscapes larger than 600 also because I think it's neat how you can scroll the image across all 3 screens.
Mac OS X includes an application called "Preview" which you can use to quickly rotate images, resize them, change file formats, etc. There is also iPhoto which is best described as iTunes for images.