There are two ways of inserting images into a wiki page: drag-and-drop and using the insert menu.
Drag-and-drop an image
While in edit mode on any wiki page, drag the image from its folder on your computer and drop it into the page which you are editing, in the place where it should appear.
Use the insert menu
On the menu, click Insert > Image.
Browse for the image on your computer, then click Insert.