Sort pics in proper numerical order

I have uploaded 500 pics into a gallery and have named the files something like Mark-1, Mark-2, Mark-3 etc. However, in the gallery, when I sort in order the files will appear Mark-1, Mark-10, Mark-100, Mark-2, Mark-20 etc etc

Does anyone have a solution? How can I make them appear in proper numerical order?

Thanks

http://wordpress.org/extend/plugins/nextgen-gallery/