Illustrations that I have added to Wikimedia Commons and inserted in some articles:
Illustration of the electric field surrounding a positive (red) and a negative (green) charge (click for larger image). I inserted this illustration in the Electric field article.
Illustration of the electric field surrounding two equal charges (click for larger image).
Illustration of the electric field surrounding one positive charge (click for larger image). I'm not quite satisfied with this one, but I don't see any article where it would be obvious to use, so I won't improve it just now.
Including the contents of a template page on a different page (by surrounding the page name with double braces, i.e. "{{}}" and leaving out the "Template:" prefix) is called transclusion.
This allows the other page to be automatically updated whenever the template page is updated.
Besides templates, pages in the following namespaces can be transcluded:
Category:
Help:
Portal:
User:
Wikipedia:
The only difference between transcluding these and templates is that you must include the prefix (followed by a colon) with the page's name inside the double braces.
Images also can be transcluded, but this is done using double square brackets instead of braces.