A handful of my friends run a convention just outside of Springfield MA for which I got to do the t-shirt design and booklet cover this year. The Author Guest of Honor was Jo Walton, so I borrowed a stack of her books and got to reading with the intent to do some imagery that would be applicable. The first scene that jumped out at me was from "Tooth and Claw", where two adult dragons (and two baby dragons) are discovering a cave full of treasure.
I had a few considerations I was taking into account when laying out the image. One was that on the booklet cover, at just under 8.5x11 inches, there would be a four inch logo covering up a section of it, and it would be all grayscale. I decided to leave the upper right corner available for the logo (essentially like how the t-shirt ended up, bellow). The other consideration was that the image would need to reduce well to only three colors for the t-shirt printing process (two ink colors plus the color of the t-shirt).
I used the opportunity to try a new method of painting, as well. I covered the canvas in thinned out burnt umber, and drew out my highlights with a rag. The technique lent itself well to the dark imagery, which in turn lent itself well to a black t-shirt design, black being a popular t-shirt color at this event. I would like to improve my ability to use this technique. I found it hard to bring back the white of the canvas, which meant I needed to go in with titanium to brighten up my highlights, which killed the colors I was getting. I'm guessing that if my rag had been lightly dipped in mineral spirits, it would have worked better for that (but I try to keep my solvent level fairly low).
I like the overall result. And it gray-scaled nicely.
I submitted two options for the t-shirt, one with and without background, due to some input I received that the background was distracting.

Lucky luckily suggested simply adding another color or two to keep the background there but differentiate it, which really improved the design. (And in retrospect makes me wish I had toned down the background for the gray-scaled booklet cover as well.)
The t-shirt inking was not as precise as I was expecting/hoping, but I like it.
