assuming the government's already chopped off the corners when you replaced them, you can just throw them away. Wouldn't hurt to just cut them up a bit more though, like an old credit card.
Some of passport photos, from the past are absolutely hilarious, so that's why I keep them all in one of my man drawers.......In one of them, I try to look like Don Johnson, in my dress sense, with obligatory long hair style, when I was going through a seriously naff Miami Vice identity crisis! :-)
I can't bear to throw them away. Every now and then we come across them and have a trip down memory lane. Also helps when you are trying to remember what year you went somewhere.
I think, if you're having a clear out and they never bothered you before, just burn them, if you don't have an open fire or stove, put them through a shredder or something