I am not at home right now. So i can not look up which picture that is exactly. But there is only one bonus picture that is a bit more tricky to get. So I think i know which one you mean.
Basically you have to sleep with Tails first, then go and talk to Sonic.
He will make a comment about Tails that will unlock a special picture for the Tails gallery.
I hope my guess was correct haha.