I don't mind if you add your own text to the icons as long as you're doing it for yourself and yeah, not for mass use. :) I only have a few textless icons up (well, more for the tenipuri ones than the death note ones), so I don't really know how much this applies. As for credit...I suppose if you do edit the icons, you can put something like original icon by lucathia_rykatu, text by yourself, in the comment box for the icons.

I hope that's not too complicated!