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 subfolder. This folder is not created by default, so you need to create it yourself the very first time that you need it.