The Image Widget allows you to display an image on your WebUI page. To do so, you have to specify the Contents option on the miscellaneous tab in the option editor of the widget. Here you can specify either a literal name of an image file in your image folder, or a string parameter in your AIMMS model which contains the name of an image in this folder. If you specify the literal name of the image file, make sure not to use quotes around the filename.
The images themselves should be stored in the resources/images sub-folder. This folder is not created by default, so you need to create it yourself the very first time that you need it.