I have checked out the editor mentioned by panzer (Chasys Draw IES) and it looks like a quite powerful tool and it does not use much system resources. It seems it used to be a commercial software in the past and now it is free. It may not be exactly easy to use, but that is more or less the rule with more powerful image editors.
Coincidentally, a new GIMP with single window interface was released today. From the screenshots, it looks like a definite improvement. Not sure if it can be considered light-weight though.