Downloading source images

It is surprisingly difficult to download an image (picture) present in a Google document. First thing to understand is that once you upload a source image, you won't be able to later download the original. You will download a modified version of it (downsized by Google). Thus it's important to keep the source image if you want to reuse it later.
To download a downsized version, the easiest trick I have found is to temporarily publish the Google document as a web page, then load the web page in a browser and save the image to disk. To publish, use File -> Share -> Publish to Web.
You can remove the publishing just after you saved the image file. However this method probably only works if you have full rights on the document (as you need to be able to publish it).

Google Docs