The approach of converting a PNG to SVG is not really good one, since PNG is a bitmap and SVG a vector based graphic. In your case the PNG (bitmap) has been probably embedded in the SVG file and therefore not imported. Signavio does not allow using PNG as a custom graphic, since the bitmaps are not scalable and could be potentially unsafe.
All you can do with a PNG image is to try to convert this object into paths using graphic processing software (e.g. Adobe, Inkscape etc.). But there is a trap in this converting method. So generated SVG could be much bigger then then the original PNG and you wold not upload it since there is a limit of 20KB per SVG file.
There is only one conclusion: create a graphic directly as SVG without using any image.