Wanted to report/discuss a possible privacy concern and image glitch, and understand it better for submission to Apple's feedback page: the iPad (and maybe iPhone) Photos app will consitently resurrect long-deleted photos in your Camera Roll, under certain circumstances. These are long-removed photos (as in removed many months ago, prior to the iOS 5 update), and possibly only the reduced-size thumbnail versions.
Note: as near as I can tell, the deleted photos were NOT originally from the camera roll! That's where they resurface, but I believe they are photos from other albums, maybe once which used to by synched to iPad but which I no longer synch. So "removed" might mean deleted or it might mean "no longer synched to iPad"... the images are so old I can't recall.
So the bug, I'm guessing, it two-fold:
1) The Photos app (and/or iPhoto during wired synch?) is not fully removing photos (or at least, their thumbs). They're no longer visible in Photos app, but are being retained somewhere on the device.
2) A glitch in the Photos camera roll PNG handling (which I think may be new with iOS 5) causes those old images ro re-surface when you add a PNG to the camera roll. (The old photos were JPEG, but the image that triggers the bug must be a PNG with alpha channel)
I have seen this on both an iPad 1 and iPad 2, running iOS 5.
Here's how to reproduce it:
1) Synch some photos to your iPad, then deselect them for synching and re-synch (so they're not there any more). Also, add some photos or screenshots to the camera roll and then delete them. I say to do both because I'm not 100% where the old photos come from, and I'm hoping this covers all bases! Actually, most people have already done these things in the past, and can probably skip this step.
2) Generate a transparent PNG using the art app Procreate (and probably any app that saves PNGs to the camera roll with alpha channel). All you have to do is scribble somethign into a blank Procreate canvas, then export it to your photo album. There is no need to do anything special to get an alpha channel or PNG format: you always get that. (Unless you used the Fill command to completely cover the canvas.) Now in the camera roll there should be your scribble on a transparent background. You can email it to yourself and see that clearly in Photoshop or Preview.
3) Here's the glitch: look at your camera roll (thumbnails view) and you'll see that the alpha-transparent background of the image has become one of your old removed photos! Tap it to enlarge and the alpha becomes black (which is fine, although it may seem odd if you intended to paint onto the default white canvas). But for just a moment, before you see the black, you will get a full-size glimpse of the old photo. (Or, maybe, an enlargement of the thumbnail?)
4) If you keep saving more alpha PNGs, you'll keep seeing more and different old photos coming back to life in the alpha regions of the thumbnails (and, briefly, at full size).
5) If you delete all those PNGs and then save some new ones, you'll see the same sequence of old photos reappear again.
Needed fix: removed phtos/thumbnails should be properly purged, and alpha PNGs should show simply black (or some default background) at all times.