You can also try this trick.
When you are in thumbnail view, select the Action icon (box and arrow) then, tap and hold on one photo with two fingers (a little over second works) then drag across the other photos you want to select. It works very well for selecting a single row, but sometimes has problems when you drag up or down.
I recently (yesterday) deleted about a hundred photos out of Photo Stream using a combination of this and the usual tap to select (depending on groupings of photos). Took about 5 minutes. Still not fast, but better than nothing.
Hint. Deleting every two or three screens means you wont' have to go back to the beginning if something goes wrong.