I have recently started teaching an undergraduate course at Dalhousie University in Halifax. The regular professor is on sabbatical, so this is a part-time gig, and a one-off. It's hard work, and shockingly poorly paid, but a lot of fun; I'm fortunate to have a fairly small group of bright, motivated students. 

One of the things that's surprised me is how little decent-quality and openly-licensed material there is on the internet for teaching technical courses like this. I can find images as well as the next person, and 'fair use' is acceptable for teaching I suppose, but often I'm left with a low-resolution image that doesn't quite show what I want. Thus I'm creating a lot of stuff from scratch, which is fine because I enjoy it, but it's time-consuming and, besides, I may never teach this course again.

So... I am uploading the drawings I make to SubSurfWiki.org, where you can find and download them, and use or abuse them for whatever you like without permission (they are all licensed CC-BY so you only have to give attribution). They are in Scalable Vector Graphics format, so you can edit them with a vector graphics tool like Inkscape or Adobe Illustrator. 

Note: There are some issues with displaying SVG files in some browsers. They sometimes look weird or even broken. You should be able to download the files and use them in a vector graphics tool without any trouble. The only other option is to use the Portable Network Graphics files instead, as I often upload those too; look for the same name, with a PNG extension.