For Magic's 7th Edition core set, Wizards of the Coast decided they wanted new art for every single card they were going to put into the set. Tristan Elwell was one of the artists brought in for the set, to re-envision the old cards. The cards he illustrated for the set were Elvish Piper, Memory Lapse, Merfolk Looter, and Staunch Defenders.
The Magic: the Gathering cards illustrated by Tristan can be viewed here: (http://gatherer.wizards.com/Pages/Search/Default.aspx?action=advanced&artist=[%22Tristan%20Elwell%22]) Note that the gatherer database does not usually show promotional cards. The gatherer can also experience difficulties with links like this. If you are taken to an error page, click on "advanced search" at the top of the screen. Once on the advanced search page, scroll down to the artist search bar, input "Tristan Elwell", then click "Add," and go back to the top of the page to click "Search." You should arrive at the page linked above.