Since last summer, Inkscape can directly do some raster modifications of your images inside your Inkscape files.
But unfortunately, sometimes you need to do more than this. Before this feature, the procedure was to first find where your embedded image was stored, open it in your favourite bitmap editor, save it, and reload your Inkscape canva.
Now you just have to right-click on your image, and select “Edit externally”. Your pre-configured bitmap editor will then open your image. After saving it and going back to Inkscape, the image is automatically reloaded.
Check all this in the following screencast.
Inkscape Labrat by Steren Giannini is licensed under a Creative Commons Attribution 3.0 Unported License.