You can use Cargo as an image host for images that aren't directly related to a specific project, like for backgrounds or hover states.
Create a page in your project list - since it's not going to be displayed in your website, you can keep it turned off. Upload any images that you need into that page. Once uploaded, all of your images will appear as small thumbnails in the "Upload Images" section of the project editor.
You can reach the original image at its full size by clicking on that image's thumbnail in the project editor. The full-size original will open in a new tab or window. Copy the image's URL from the address bar, and use it anywhere in your website. The image's URL will start with "payload":
If the image URL starts with "transit", the image has not yet made it into the Cargo content delivery network. Close and re-open the page in the project editor to try again for the correct URL.