My oldest icon is one of Snape that I snagged somewhere before I learned the LJ culture of crediting icon makers. So it has sat for five years now with a note that I need to figure out whom to credit. I have no hope, I think, but I love the icon too much to give it up.
I swap icons around freely. Periodically I free up slots by deleting non-BTVS fandom icons. For instance, the BBC Sherlock icons are on the chopping block next. I'll keep one to use for rare posts & comments about that fandom, but otherwise they're gone soon. None of them are pretty enough to make me need to keep them.
There are several other stalwarts in there that I'll never give up or change. Rupertus Domesticus is there for good.
These days I am careful when assigning a tag to a new icon. Tags are something that stick around. I prefer to reuse existing an existing tag rather than delete one and make a new one. All the posts and comments that used that tag will get my default icon! Woes! So I pick mood words or topic words that seem useful, and swap them out when I get tired of an icon or find one that fits but suits my taste better.
In other cases, I have icons with descriptive tags that I end up consistently using with certain post topics. For instance, I have an icon of Granada!Watson with the text "likely stories" on it. I use it for every writing craft post I make. It's a lovely image of Watson writing in a journal and smiling. Writing craft!
Fandoms represented, in order of frequency: BtVS, Doctor Who, Sherlock Holmes (various incarnations), Top Gear, Harry Potter, Star Trek TOS, the West Wing, the Forsyte Saga (1967).
Giles pairings represented with multiple icons: Giles/Buffy, Giles/Xander, Giles/Ethan, Giles/Jenny, Giles/Spike.
Giles pairings represented with 1 icon: Giles/Angel, Giles/Anya, Giles/Faith, Giles/Joyce, Giles/Olivia, Giles/Oz, Giles/Wesley, Giles/Willow.
I use them in relation-shippy as well as friend-shippy contexts.
Oddest pairing represented: Core 4 Scoobies.
Most-represented icon maker: 10000_pixels.
Other favorites: katekat1010, schattengrafik, literati, ruuger, glimmergirl, harmony033, wickedfox.
I sometimes make icons but I'm not very good at it. It's harder than it looks. My favorite of the ones I've made is the one above of the Second Doctor. I also like this one of Giles & Xander. For reasons that are hard to explain I've always considered it a friendshippy icon not a pairing icon. I would use it for anything exploring the father-son aspects of their relationship.