These avatars are only available annually, usually only 1 day! Here's a quick list for you to keep track of what day you need to get them.
I *heart* Sloth
Visit here on January 14. The answers are C D B C A.
Complete one of Jhudora's Quests on February 4.
Send this valentine to someone on February 13 OR 14.
Visit this Neopedia article on March 4.
Complete one of Illusen's Quests on March 17.
Visit the news page (after the prank is taken down) on April 1 (April Fools Day).
Send this Neogreeting to someone on April 8.
Go to this Cooking Pot and mix together Blumaroo Steak, Orange Juice and Gourmet Cooking for Your Pet anytime during August.
I recommend buying Gourmet Cooking for Your Pet BEFORE August because the book tends to inflate a TON.
Visit the news page on November 15.
Visit the Advent Calendar on December 1.
Seasonal Attack Pea
Awarded on a random day in December when you visit the Advent Calendar and collect your prize.
I was thinking of making a guide like this. Great job! +thanks/rep.
You must spread some Reputation around before giving it to Kristin again.
Glad my guide was inspiring
You could also include the altador cup avatar in this since that comes annually.