What type of screws and rawl plugs should I use?
3 feet high, 2 feet wide. Can't weigh more than 30-40 pounds, and that's on the high side. I personally think you can get away with one good anchor. Once you go to 2, you want to have them dead level with each other, or you'll have to battle the alignment monster. 

Get a 1/4 lag bolt shield and a 1 3/4" long lag bolt, get the proper sized masonry drill bit (it will be specified on the packaging) and make sure that you account for the depth of the plaster when you drill. Probably should drill a total of 1 5/8ths deep from the surface of the wall. Then tap the shield in as far as you can get it and tighten the lag down. Just leave a small gap between the plaster and the bolt head when you tighten it. Then hang the string right in that gap. That should be good up to about a hundred pounds WITH safety factor applied. Failure point is about 200 I believe, over 4x what I'm assuming the weight is. 

I just realized you're metric. I don't know enough about metric lag bolts and thread sizes to convert what I've said, but perhaps you can get the gist of it.