When ds1's lovely teacher left, we did a collection and got her a picture frame, but we also organised a scrap book where every child in the class drew a picture of themselves and wrote a message for her, and she really loved that.
I was the one organising it, and it was a bit frustrating, chasing up the pictures, but well worth it.
A few years ago I took over from a teacher. I cut out leaf shapes, and had the children write a personal message on them. I then drew a tree trunk and glued the leaves all over the paper to make it look like they were on the tree. I then mounted it in a frame. She absolutely loved it.