I have two candidates.
My second choice will be first: Lower Rock Lake off the Serene Lake Trail.
Don2
This picture was taken either from the mystery trail or very close to it.
The trail itself is not on the official list but is well used as it leads to unofficial campsites.
There is a nearby trail that is on the Forest Service maps.
Bryon
It's the Twin Lakes trail. The campsite in the photo is on the edge of Upper Twin Lake. Your turn, Bryon.
The fire did not reach Upper Twin, although it came darn close. Lower Twin was burned around two-thirds of the shoreline. There are some photos of the area on my Flickr site if you're interested: http://www.flickr.com/photos/19191522@N06/sets/72157631755298438/
And I don't remember much in the way of river rock near Welcome Lakes. There is West Welcome Lake, but its a bit off the trail. And the small lake next to Welcome Lakes Campground may be unnamed on the maps but is swimable. So either Welcome or Twin could be it.
Bryon Boyce said
Twin Lakes
I think it is either that or Welcome Lakes, but given the clue about two places to swim, I think you are correct. I haven't been to Twin Lakes since the fire. It looks just as bad as the are around Welcome Lakes. (if that is where those photos were taken)