Inkscape is not a raster program, so it won't save it as PNG or open your png files for editing! To save as a png, you will have to export it, using either menu > export or the export toolbar icon. To open a png in inkscape (or any raster image), you have to import it into inkscape as an object.

I suspect that when you save it as ".png", you are saving it as a svg with a false .tag! When Inkscape tries to open this, it crashes; explorer doesn't make thumbnails out of SVG files; and GIMP actually converts your SVG file into a raster file for editing.

Btw, exporting from Inkscape saves it as a 24-bit PNG, so if you want it indexed, you need to open it in GIMP anyhow.