First you have to upload your image to Wiki.
There are many options on how the image is displayed. Please read documentation to learn the details.
The easiest option is to use the following syntax: [[Image:imageName]], where image name is the name of the file you uploaded.
For example [[Image:tornado.jpg|50px]] produces an inlined image. This example also demonstrate how you can control the size of the displayed image (the width of it while preserving the aspect ratio, to be precise) in pixels (px).[[Image:tornado.jpg|left|200px]] produces an the image on the left side of the screen with the text wrapping on the right