I was asked to provide 2-3 rough concepts for the cover. Sometimes IDW cames back with a feedback or asks for another sketch; once they are happy with the concepts they send them over to Hasbro; Hasbro then selects one cover and sometimes requests small changes to be made.
Last time IDW sent me a manuscript to read and draw something from it; other two times they gave me a free hand, so the RI covers are often "random".
I don't have my drawing tablet with me right now, so to make those covers I have dusted off Inkscape -- vector graphics software I have used to create all my art and comic prior to "A Storm's Lullaby".
My first concept is Tempest in her Season 9 finale gear :)