I have transfered pictures from my PC into the Xoom Gallery app via USB cable. When I choose a photo and delete it, the picture immediately dissapears from the gallery list. After rebooted the Xoom, the deleted photo appears again.
If you have that hooked to your gmail account, or set up Dropbox or Picasa on your account, those pictures will be synced and re-shown on your device. You can try the QuickPic app for deleting photos without this issue.