You would repair them, the same way you do drywall.

Google drywall repair. Also HD, Lowes, etc have DIY books that cover this subject.

Here is a link that might be useful: Drywall repair

Thanks for your help.

Sorry I guess i failed to mention they're definitely plaster, not drywall. So since we have large sections (i.e. the entire length of the walls where the baseboards were removed) it would be difficult to remove a section and replace it like you would drywall.

My main questions was concerning whether different types of plaster walls (plaster on gypsum vs plaster on wood lath etc.) need to be repaired differently or if its pretty similar?

It's all pretty similar ... but get a good "bonding agent" and use it in the shallow divots where picture hangars came out, then patch.

Consider installing picture rails so you don't have to pound nails.

Usually you moisten the edges with the bonding agent, then apply the patching plaster, smooth it out.

I've patched a lot of plaster in our house! We have plaster with horse hair on wood lath. I can't tell from the pictures if you actually have holes that show the lath (or other substrate, they also used metal I believe). If you're not a purist and you have holes you can patch them with a bit of drywall cut to shape. You can screw that into the lath, if possible, or just glue it in. If you're a purist you'd have to go to great lengths to find the materials to make your own plaster for the 3 different coats. When I found out how difficult it would be to source materials to make my own plaster I started patching with drywall as needed.

It doesn't look to me like you have actual holes, though. If you're just missing the nice finish layer of plaster you can use joint compound. If you want a hard, fast drying patch then you should use a 20 or 40 min joint compound (but you have to work it quickly since it dries quickly!). If you just need a light coat you can use regular joint compound that comes in the big buckets. It will be pretty soft, though, and not able to stand up to much wear, so definitely go with the 20 or 40 if it's a high traffic area (especially on corners).

If you can't find the bonding agent the poster above mentioned (I've heard of it but it's hard to buy/locate unless you're a contractor-- sometimes you can purchase it from a local plaster repair contractor), you can try using a high quality primer, instead.

Attached is a link to a quick tutorial on plaster repair. As long as you get a slower setting plaster than the stuff billed as "plaster patch" at the big box stores it's not hard to do. In New England the big box stores all carry "real" plaster supplies, as that's still the norm here. You may need to mail order or find a real building supply place in SoCal. (Look for a USG or National Gypsum dealer in your area).

Here is a link that might be useful: how to link

This problem is completely being blown out of proportion.

You can easily repair cracks / chips / small holes with plain 'joint cement' and paper or fiberglass tape.

From your photo - it looks like surface damage. Forget about mixing plaster.