I steal 'em!!
Here's a page from BoatUS about boat plumbing. There is a graphic on the page of the plumbing schematic. I hover my mouse over the graphic, right click and 'copy image location' from my Firefox browser. Then I paste the image url between [img] [ /img] to display the graphic directly in the post, like this:
(BTW, my plumbing system looks just like this, too!)