In order to place images on any NationBuilder page you must first upload them to the page (this also applies to email blasts). From the Settings tab or the Content tab for that page in the control panel go to the Files tab. Any Files tab in NationBuilder allows you to upload images from your computer directly to that NationBuilder page or email blast. 

Once you've done this you still have to place the images where you want them on the page; you do this in your content editor, so go back to the content tab (or corresponding tab that brings you to the page's content editor). On your content editor's toolbar you're going to click on the Insert edit/image button, the button which looks like a window with a green plus sign in the bottom right hand corner. When you do this, you'll get a insert/edit image pop-up on your screen. The second field down is Image list, from here you'll be able to choose from any images that you uploaded in the files tab for this particular page. 

You can add an image description, which helps with search engine optimization. Also, there are lots a fields for adjusting how your image looks on the page. When you are ready click insert. You can always resize your image in the content editor too.