I like to give credits as a way of thanking people for particularly nice items they have given away, but I don't feel obliged to do so every time I use it or for everything I use (cataloguing the creator of every texture would be a nightmare). If people use props of mine in a picture and give me credit, I like that, but if they don't, I don't get worked up about it either. I'm just happy that my props were of some use.