There is a rather good open source vector image editor DIA. It can be used to create some diagramms to computer-related atricles. This format is easily convertable to .svg, but after the conversion it can't be easily edited (in dia, for example there can be a linked objects like in visio, when you move one object, linked arrows will move too. after conversion to svg this feature is lost).
Please, allow users to upload images in .dia format.