From your description, it sounds very much like you're living with plaster and lath walls. If I'm correct, there are two possibilities for solid fastening. 

 1. Find the studs. They are probably 16" apart, so once you find a couple, you can probably find more. Stud finders in those walls are fraught, as there's so much wood and metal to confuse things. If you already have holes, you can stick a bent clothes hanger in and spin it to find a stud. (Unless, ha ha, you have insulation in the way. Then all bets are off.)

2. Get a picture rail high around the perimeter and hang the art from it wherever you want. You still have to find studs, but at least it's a one-off exercise. Plus, it's very much in keeping with the 1904 aesthetic.