Internet sources seem to have a fleeting existence. Whenever I can, I print the webpage as a pdf, or down load the image. For those sites that do not permit images to be downoalded, I take a screen shot. There is always a loss in resolution, but it is better than nothing.
Secondary sources that you mention can provide valuable clues, but try to verify the info against real sources whenever possible. Sometimes a secondary source will provide a number of actual sources, I suggest reviewing these for relevancy, I have seen situations where the sources have nothing to do with the data they are supposed to verify.