Go to Wikimedia Commons and find the image you want to use. Click on the Wikipedia logo that says "use this file." Copy the location of the thumbnail file.
Go to your article and hit "Edit." Go to the Source Editor. Paste in the link you copied before wherever you want the image go to. Hit Publish!
Edit the caption by going to Visual Editor and clicking on the image. Click "edit" and change the caption. Hit "apply changes" and then when you're in the regular visual editor again, hit "Publish changes."