There are the typical ideas of using the birthstone ponies, one for each month...you could have the birthstone doing a commentary, kinda like a news report, split picture, talking about what is going on in the other part. If you want just a single picture, as in you don't need to work with Photoshop or any other utility, you could probably still do something like that...Birthstone in the corner, 'watching' the others.
January--party for New Years
February--Valentine's party/love theme
March--could be anything really, but you could go green for St. Patrick's Day
April--I'd use Sunshower/Luau, with their very FLOWERY symbols for April Showers/May Flowers idea
May--Mother's Day, so Moms/Babies together
June--Father's Day, scenes for the 'boy' ponies!
July--4th of July picnic type scene
August--BACK TO SCHOOL!
September--Labor Day, so maybe some type of parade?
October--Halloween, duh!
November--Thanksgiving/pilgrims & NA scene
December--HAPPY HOLIDAYS!
Of course, you can make her birthday month just a big birthday celebration picture, no matter what "holiday" may be there. Just make sure everyone has their clothes on! No adult themed MLP calendars!