Is this image of takatsuki_yayoi or of someone else? I get that it's her birthday, but I'm not really familiar with アイマス.

アイマス is just an abbreviation for "Idolmaster". You had it right--this pic is tagged on Pixiv as 高槻やよい, so Yayoi she is.

Oh, I know what Idolmaster is (and its abbreviation, obviously): I did manage to tag it. But this character doesn't seem to usually have her hair down and quite so...orange. So I asked to be sure.