Very slick.
Dont kid yourself... A great deal of the Graphic Designers out there do just what you did and call it theirs. I had a prof. in college (for graphic design) that used to say that the real skill was in bringing the elements together in a harmony that speaks to people.
At the very least please email the artist.
I'm a FT graphic designer and college professor teaching intro digital art classes part time - this is a very touchy subject for us and sometimes very hurtful when our hard work is devalued from the public.
Hopefully, your instructor wasn't referring to stealing images, but was instead referring to finished designs, pieced together from legit sources using natural creativity. Nearly
word for word, I say things very similar to what your instructor told you, but I wasn't referring to found artwork from someone else. In the classroom, images can be used and found online for educational purposes. Out of the classroom, using those images in most circumstances is illegal. There are images out there that exist solely for your use, they are free and you can download them and use them wherever you want. These are images/clipart/photos that graphic designers use as a resource and we know they are free for us to use in the real world. I have plenty of sites bookmarked and don't mind sharing my resources. Stock photo and artwork sites cost $3-$5 an image if you choose to go that route.
The other side of it - and please be ethical - is taking someone else's artwork that they created/slaved over. The "villian" image you have there is beautifully made. And your label is beautiful as well. The least you can do is show the label to the artists, request their blessing, give a very small "artwork provided by" shout out on the label or just give them known credit and a "thank you".
As a designer, I would be happy to find you a licensed free version so you can avoid taking imagery from this artist's portfolio if they don't approve your use. The artist holds a master's degree and I assume he makes his living by selling files and prints. DeviantArt is a medium most of us use to show potential clients our work, get feedback from other artists, network, etc. What some borderline unethical designers do is they will take an image from someone else, and then edit it so much that it is virtually indistinguishable from the original.
On behalf of all designers out there please be ethical.