Sam and Max creator and Monkey Island artist Steve Purcell has posted some nice concept art on his blog he created back in the day for Monkey Island 2 (also published in Rogue Leaders) and some very early concept art for the Curse of Monkey Island.
Check out that awesome flaming beard LeChuck and a nice slender version of the Voodoo Lady:

Purcell also blogs:  ‘I have a rough sketch for the Tales of Monkey Island pre-order cover in the pipe waiting for final approval. I’ll post it at some point soon.’

We’ll definitely keep an eye out for that one!