Once you’ve uploaded an image to the Commons, adding it to a Wikipedia article is very simple. You just go to the article, click “edit this page,” decide where you want the image to be, and type a link to the image page. It’ll look something like this: [[Image:Nameofimage.ext|some other optional stuff]]. (More on the “other optional stuff” in a moment.) Add an edit summary, do a quick preview, and save the page.
The software assumes that the image name you add to an article refers to one at the English Wikipedia, if one exists there. If the English Wikipedia doesn’t have an image with that name, the software looks at the Commons. That’s another good reason to name an image at the Commons in such detail that it’s unlikely you’ll accidentally choose one that already exists at the English Wikipedia. (No, the software won’t warn you.)
Forgot the name of that image that you uploaded to the Commons? Don’t worry. Just sign in, click the “my contributions” link in the upper-right, and look at the names of the image files you’ve created. You can do the same at the English Wikipedia, if you’ve uploaded images to that site. Filter out everything but image pages by changing the Namespace field to Image.
When there is more than one image in an article, spread the images evenly within that article. Each image should be relevant to the section it is in, and normally there should be only one image per section.
For accessibility and other reasons, you should ...