4 wooden battens, screwed to the wall horizontally? How about a frame, instead of the plywood?

A vote against drilling all those holes in the wall; it's going to be difficult to get it all just so. And if you make a mistake, it's a bitch to undo.

Would these work? They are very strong.

The concrete is painted, (landlord off-white! Buy it by the tanker truck-full!) and those stick-ums would probably pull the cheap paint off the wall.

The stickums work well. And if they do pull the paint off, they're cheap enough that you can just put another one on, over the now-unpainted area where it will stick.

It may be that your idea with the plywood turns out to be the easiest, tidiest way to go... If so, a couple of pages that may help: Drilling in Concrete, and Mounting Wood to Concrete. Just be sure to use the proper wall anchors, correctly installed and it should be perfectly safe.

I just typed "he should be perfectly safe". The mind is a strange thing.

What about a single nailer (like a 1x3) up high, to which you can add nails and then hang the pics with fishing line?

I had to hang a bunch of 8x10 clips frames just like that once, in 2 rows of 8. I put 8 nails in the nailer, cut 8 equal lengths of fishing line w/ a loop on each end for the top row. For the bottom row, I made 8 shorter but equal length line pieces, looped at each end, and ung the bottom row from the bottom clip of the top row. Wokred like a charm and held fine.

I used heavy clear fishing line (I was paranoid!) and regular chrome crimps to make the loops. As long as you get the nailer up top level and the space right on the first row, you should be good to go.

Oh, I looped the fishing loop inside the clips on the frames, if that wasn't clear how it all went together.

And you could even paint the nailer black to artify it.

I would do chewy one better by putting a wood strip at the bottom, as well. Run the lines between the two, and get them tight.

It is not too hard to put a masonary nail into concrete. One sharp blow should do it. Amazingly, there only needs to be a small length of nail in the concrete to hold well. Drill holes thru the wood for the nails, line up, then *WHAM*.