When selecting from the main view (long-pressing, checkboxes appear), there should be a trashcan icon on the top for you to erase the pictures. When viewing them individually there should be a similar icon on the top already or if you hit the menu button, the Delete option should be there (unless this is from a synced source such as Picasa, then I don't think you can delete those, you'd have to unlink your Picasa album from the phone)