I set up three carousels, one for me, one for a group of friends, and one for family. I invited my wife to the family carousel when she joined, she got to see some of the family pix but also the pix from the friends carousel. Not a real problem for me but irritating.
Is it possible you have enabled Auto-Import in Revel Settings on your iPhone or iPad? If so, any new pictures that are saved to your Camera Roll will be uploaded to the carousel which you specify in Revel Settings. This setting automaticallyu imports all new pictures added to your Camera Roll, eg., saved from email, shot with you iPhone/iPad2 camera, or added by Revel if you choose Save to Camera Roll when saving your friends pictures to your device.
Also, your friends can specify your shared carousel as their own Auto-Import target--but only the carousel you invited them to (so I think this is not your case, unless your wife is also added to your friends pics carousel).
The first scenario I mentioned is the only way I can reproduce the kind of behavior you describe. Otherwise, I do not see images in one carousel spontaneously appear in another carousel. If this is indeed what's happening, and you've struck down Auto-Import as the cause, please post back here to spur further investigation.