I've been using an awesome free vector graphics app called EazyDraw (definitely check it out if you need to do any technical illustrations). The only drawback to this app is that it does not export to GIF. The graphic in question is the sketch of a math function on a coordinate grid on a white background. However, when I open the result in Preview.app and export to GIF, the white background becomes some light shade of grey. This doesn't happen if I export the image using GraphicConverter, but I prefer Preview.app because it doesn't force me to wait 30 seconds on startup. Does anyone know some clever trickery to make Preview.app export better? There aren't any options in the Save as... dialog.