Ok. Maybe this will illustrate what I mean with Home Sharing. These screen shots are from iTunes on my Mac, but it should be very similar on Windows.
Turn on Home Sharing in iTunes.
Then select the option to choose photos to share with Apple TV.
Attachment 40373
Select a folder/folder to monitor.
Attachment 40374
Keep the iPhone that is taking pictures attached to the PC. Use windows explorer to copy photos into the folder select above to monitor.
Make sure Apple TV is logged into Home Sharing.
Then go under Computers.

Select the PC.
You should see an option for Shared Photos.

Select it.
Then select the folder that you shared.

You'll see thumbnails of all the photos. There is a link in the upper right for Slideshow.

Select it. Then you'll get the slideshow options and then select the Start Slideshow button.
This does work as those bad Apple TV screen shots (taken with my iPhone) were taken after I turned this feature on. No Internet required. The part I didn't test was adding photos to that folder I chose to share.