"...the labyrinth was empty. Dean had disappeared."

Okay, time travel fans, The Blackhope Enigma by Teresa Flavin is a good one for you.  Dean's just goofing around as he walks through the labyrinth in Blackhope Tower, but poof!  He disappears, and his sister Sunni is shocked to see him IN a nearby painting, done centuries ago by the mystical artist Fausto Corvo.  So Sunni goes in after him, as does her classmate Blaise and (uh-oh) bad guy Angus.  Soon they're crossing through different layers of the painting, ending up in different lands, encountering some nasty creatures, meeting others "inside" the painting, and having one heck of an adventure as they desperately try to figure out how to get back home. Who can be trusted?  Parts of the story were a bit confusing, but many readers will enjoy the pace and action.