The approach of converting a PNG, JPEG, GIF or BMP to SVG is not a good one, since a pixel graphic format differs quite a lot from a vector based graphic (SVG). In your case the graphic has been probably just embedded in the SVG file and therefore not imported. Signavio does not allow using any cause PNG, JPEG, GIF or BMP as a custom graphic. The cause are e.g. the bitmaps are not scalable and could be potentially unsafe.
All you can do with your original 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 than the original image and you would not be able to 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.