The walls look and sound like plaster, probably over wooden lath. The lath is then nailed to wooden studs at intervals, probably at 16 inch centers.
A common approach to hanging art and mirrors on both plaster and plasterboard (drywall) is to use picture hooks with thin, hardened nails, such as these
The nails are sharp enough and thin enough not to do damage to most plaster. The hooks (properly sized and in multiples) are strong enough to hold up over 100 lbs. Canvases on stretchers should be no problem unless they are many feet long.
Use at least two hooks to spread the load and improve leveling of hte pictures.
When the load approaches 100 lbs., extends out from the wall, or is subject to movement, it's time to find studs, or at least heavy duty toggle bolts.
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