Back in the day, retailers used to commission star comics creators to draw a cover for an upcoming comic book, usually from DC or Marvel, that would be exclusive to their store. Each store would have to order thousands of copies to make it worthwhile for the publisher.
Some enterprising artists realised they could commission themselves to draw exclusive covers of such comics, get the publishers to print them, and then sell them online, or at shows, signed or unsigned, for a considerable markup. J Scott Campbell, Rob Liefeld, John Tyler Christopher are amongst those creators who have done so the most, and soon the habit spread.
At Dynamite Entertainment, which asks for fewer minimum copies to be printed than Marvel and DC, Dan Panosian did such for Red Sonja #1 as did Paolo Pantalena, as have other artists for different titles.
The latest, it seems, is Ethan Van Sciver. No longer working for major comics publishers and fresh from last summer's very successful Indiegogo campaign for Cyberfrog: Blood Honey, the comic is yet to be published. But Ethan is now commissioning himself to draw Cyberfrog variant covers to Dynamite Entertainment titles, showing his cyborg amphibian interacting with other characters published by Dynamite. At one point, Vampirella and Cyberfrog were both published by Harris Publications, and Ethan has done artwork for Vampirella trading cards and inked the Vampirella/Shadowhawk crossover in the day, so this wouldn't be the first time he's been involved with the character. But I hear there are more to come, one might consider Chaos!, Pantha and others as prime Cyberfrog crossover comic book cover candidates.
Traditionally, creators who purchase the comics are treated the same as any retailer. Whether the creators of any of those books will have any say in who gets to do retailer exclusive variant covers of their books, I don't know.
But basically that means that Ethan Van Sciver has bought hundreds of copies of the same comic, with a cover he drew, for a wholesale price that retailers pay, maybe aroud two bucks each, and gets to sell them at quite the markup. Well, it's one way to get a Cyberfrog comic out there for convention season, draw the character on the cover of someone else's comic and sell that…