Alright, it's about time I asked...what's with all the Madoka/Seal, Homura/Otter and Sayaka/Llama pics? Is it a play on how their names are similar to the Japanese spelling of those animal names?
For Alpaca+Sayaka, she was inspired by this MAD: http://www.nicovideo.jp/watch/sm19158115 Changing her famous quote from "ăăăăŁăŠăă»ăăšăă«" (Atashitte honto baka - I'm such a fool) to "ăăăăŁăŠăă»ăăšăąă«ăă«" (Atashitte honto arupaka - I'm such an alpaca)