From the Kickstarter page:
Mentor: All of the above, plus when Roll20 launches you'll receive a "Mentor" account instead of a normal account, which gives you increased upload storage capacity for your custom art uploads. In addition, even after the site has launched you'll continue to receive access to a "development" version of the site where you'll have access to new features before everyone else. Credited on the website as "Mentor."
<a href="http://www.kickstarter.com/projects/rileydutton/roll20-virtual-tabletop-gaming-that-tells-a-story?ref=live" rel="nofollow">http://www.kickstarter.com/projects/rileydutton/roll20-virtual-tabletop-gaming-that-tells-a-story?ref=live</a>
Basically, if you donated at the Mentor level or higher ($50+) one of the perks was the increased storage capacity.