i am in favour of crediting. i have also on aoccassions contacted the original artist to ask for permission. Experience telld me that a few never even bother to answer though

And indeed it's hard sometimes to tell who really WAS the artist, especiall when the image is found a on a site that does not mention artists (there is plenty of those)
Also i'm having some trouble where you would mention it. I've done so on the preview, on one occassion i've done so in the theme name. But since I also want to add 'by Vlammetje' to the theme name nowadays it's getting a bit clogged up.
And the description filed is suitable for it but won't help when the theme is leeched

Anyway... back to the original question: yes I favour mentioning the artist and I also favour asking permision... even if I already know that some artists will never reply and some will refuse without even considering

But honesty goes a long way I think.