You can just upload them as files. Use the File Uploader element.
For images, you can use the picture uploader for images of course. Or just create a datatype with an image field. The data tab > app data would let you create a new thing of that type and click on the image field to upload your image.
If you need for some reason to host files in the root directory of your site, you can do that, too: Settings > SEO/Metatags (scroll down to “Hosting files in the root directory”.
Also (more obviously), for images you use in designs, plopping down an Image element lets you get the images into your database and page at the same time of course.
That tends to be for things like scripts from various services (e.g., OneSignal requires several scripts and a .json file at that level) or authentication files (put this file here to prove you own such-and-such domain). But you could use it for anything.
For things like images, it is of course better to have them as objects in your database.