Windows PC as Itunes server and iMac as photo provider on ATV2

A NAS like QNAP or Synology.

Not sure if you can set the aTV2 to stream photos from one computer and music/videos from another. Moot point since you've indicated that you don't want to stream from the Mac.

Here's some alternatives to putting the photos on the PC.

Use Flickr. Upload the photos that you want to use as screensavers onto your Flickr account. Set the aTV2 to look into that Flickr account/folder and viola! Note - not everyone wants to put their personal photos onto flickr for various reasons.

If you have an iOS device, you may want to look into Photostream. I'm not sure how that integrates with iPhoto, but I'm pretty sure that aTV2 can hook up to Photostream.

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You set up the pictures you want access to on the AppleTV via iTunes on your PC. If you have your network set up right you should be able to point iTunes to the picture files on your Mac. The AppleTV will store a bunch of pictures and them change them out occasionally. You shouldn't have any network or performance issues on either of your computers because AppleTV doesn't access your computer constantly, it just updates pictures occasionally in the background. Plus, when either or both of your computers are off, the screen saver still works with pictures stored on the AppleTV.