I usually credit where I remember where I got it. I'm not always that good at crediting textures, but I like to when I can. I also think that the vendors can find out what images they are credited in, and I like to give my favorites kudos. About the only time I don't credit when I do know what I used is when the gallery software eats my image, and I'm too frustrated to enter it again. (It used to be that if you forgot to check the check buttons, or specify an advisory, it didn't remember anything you did.)