Crediting references

[dmh_3000]

So as a practice exercise I've been using other drawings as a reference, mostly covers, to draw some pin ups. And of course, I've been signing them all with "After so and so." However, a couple of these covers already have the "After" mark.

So, say for Batman #666, it's drawn by Andy Kubert, who signed it "After Grant Morrison." Do I say "After Kubert, After Morrison" since they were both involved, "After Kubert" since he did the drawing I'm looking at or "After Morrison" since he came up with the idea?