Hi @I_amFrk05: I hope you are using Theme-Cosmos. As mentioned by others in the comments, changing the icon to use images cannot be done from App studio. You need to switch to Dev studio to do this.
Another consideration is that- to do this you may need to do some customizations because the navigation rule used by theme cosmos (MainNavigation) is designed to get the pages dynamically which you added in the channels and interfaces. It is recommended not to customize the cosmos-design pattern. Also, it is recommended to use icon class instead of images in this scenario.
Said that, how can you add your own icons to icon class? Below references will help you.