Kidby's work on the Discworld franchise has had an enormous impact, with his wonderful renditions of the many much-loved characters being far more widely accepted by the fandom than the works of Josh Kirby, who - despite being a fairly skilled artist, himself - never really captured the spirit of the Discworld.
(Granted, Pratchett himself has mentioned that he wasn't entirely pleased with Kidby's Clint Eastwood-esque version of Sam Vimes - apparently, Sir Samuel looked far more like Peter Postlethwaite, in the author's mind.)
I'm being a bit pointlessly contrarian... but I actually preferred Josh Kirby's cover art, which I felt better captured the sheer amount of crazy that was going on at any given time in a Discworld novel. Still, my headcanon now has a mixture of Josh-style trolls (Detritus being a particularly large example thereof) and Paul-style trolls all walking around the Discworld - and I could never decide which type of troll I wanted Mr. Shine to look like.
I also used to imagine Vimes as looking and sounding a bit like Inspector Frost from "A Touch of Frost" - but with a lot more muscle on him - and imagined him wearing a raincoat and floppy hat on top of his armour. Then I decided that perhaps as a commander and a duke, he would have various markings of rank/nobility added to his armour - which he wouldn't like - and would use the hat and coat to cover up plumes on his helmet, sculpted muscles on his breastplate... Of course, he could just go and get himself some standard equipment, but I liked the look of this imaginary Vimes too much to let go of this image.
Isn't headcanon wonderful?