There are quite a few online diagram editors, but
yUML.com is not a painting tool: It generates UML class and use case diagrams on the fly from image URLs. The samples page lists a few examples, here's a more complex one:
CODE:
<img src="http://yuml.me/diagram/class/[Customer]<>1-orders 0..*>[Order],
[Order]++*-*>[LineItem], [Order]-1>[DeliveryMethod], [Order]*-*>[Product],
[Category]<->[Product], [DeliveryMethod]^[National],
[DeliveryMethod]^[International]"/>