We've always considered piston slap a loose bore.. (skits slaping the walls) Which could be possible if the pistons were installed into cylinders that were setup for forged pistons. The casted pistons tend to be more resilient against cold seize since they dont swell like the forged pistons do, hence the tighter tolerances. As long as the bore was properly setup you shouldn't get piston slap.
Pistons dont look too bad.. I've seen worse. Mic'em out and see what the specs are. WSM are good pistons. I wouldn't worry about the quality of them.. I'd be more concerned with the bore specs.