(Pssst! "RIYL" stands for Recommended If You Like.)
Here are some middle school to junior high-appropriate graphic novels, with fantasy and/or adventure elements. At both school libraries I've worked in, the students are CRAZY for Jeff Smith's Bone series. Before Scholastic started releasing the new full-color versions of them, I had already collected and read the entire original (black & white) series myself, and loved it.
When the kids finish all the Bone books, they're hungry for something in the same genre. Amulet is the most popular one, but there are others out there. These books are all selected from my library's shelves, which means there are plenty more great titles out there, but these are just the ones we happen to have available for our students.
Rapunzel's Revenge by Hale, Hale, and Hale
Amulet by Kazu Kibuishi (#1 in series of same name)
Pinky & Stinky by James Kochalka
The Royal Historian of Oz by Tommy Kovac and Andy Hirsch
The Sons of Liberty by Lagos, Lagos, and Walker (series)
Mal and Chad: the Biggest, Bestest Time Ever! by Stephen McCranie (series)
Prince of Persia by Mechner, Sina, Pham, and Puvilland
Mouse Guard by David Petersen (series)
Crogan's Vengeance by Chris Schweizer (series)
The Wonderful Wizard of Oz by Shanower and Young (series)
Jellaby by Kean Soo (series)
Kid Gravity by Walker and Jones (series)
Pandemonium by Chris Wooding and Cassandra Diaz
Also a possible fit for this list are graphic novel incarnations of some popular fantasy book series, like Pendragon by MacHale, Redwall by Jaques, and Percy Jackson by Riordan. The kids seem to like these okay, but they don't LOVE them. At least in my library, they seem to prefer the original novels. :)
Bonus "alternative format" titles (part novel, part comic)
Bone: Quest For the Spark by Sniegoski, Smith, and Hamaker (series)
The First Escape by G.P. Taylor (#1 of the Doppleganger Chronicles)
Malice by Chris Wooding and Dan Chernett (series)
DISCLOSURE: I put two books written by ME on the main list (Royal Historian of Oz and Wonderland) because I really think they fit. I don't normally do that kind of shameless self-promotion. But there it is. And isn't Andy Hirsch's cover art for Royal Historian of Oz beautiful?