First, see if you can find the image that you want to use already on the wiki. The majority of images are at least partially categorized properly, so you can search for them. When you find the image, copy the file name, with underscores in place of spaces.
From there, there's two ways to embed them:
If you're updating a gallery, just click "Add a Photo", and search for it by the copied name.
If you're adding an image outside of a gallery, just click the button to add a photo, and use the "find" feature and search for it by filename.