I like istockphoto.com a lot - it costs money, but the images are super easy to work with, etc. i usually make the design in illustrator or publisher, then i just print it out. that way i can make sure everything is aligned,
etc.
what sort of images are you looking for? not everything will work perfectly on gocco - you can't do images with lots of tiny details, for example. you also have to be a little careful about font - you can't go super small or it kind
of blurs. usually 12pt or larger is perfectly fine (smaller works too, but not as well for scrolly fonts).
there is a flickr group dedicated to all things gocco - it's been super helpful for me!!
http://www.flickr.com/groups/gocco/
Where did you end up finding your gocco? Do you have all the supplies, etc? One thing I highly recommend is ordering a bunch of extra bulbs and screens - you'll need them! If you mess up, it's nice to just have the extras there
instead of having to wait a week or so for them to come in. :)