When editing a hub, click on "Add" to the left of your text capsule (or box). You can then choose "Photo". Click on "Edit" (on the photo capsule) and a new box will pop up that gives you the option to upload one from your computer, import one from the internet (in which you paste the image url), or use one already in your gallery.
Once you upload or import a photo, it will show in the right section of the box. It will give you the option to add a caption, a source link, and a title. Fill those in and you can link to whatever page you want.
BE SURE TO USE IMAGES LEGALLY! There is a great article about this in the learning center (link to this is at the bottom of the page). Hope this helps!