You can either use an image as a background image and set the background image position set to Cover, or you can use an image widget, inside a container that is set to relative position and give the image a width of 100%. I recommend using images with aspect ratios that are the same as the aspect ratio in the section/container/div etc, where they will be used.
Same goes for sliders.
Images can be stored within in Webflow where you also can grab it’s unique link via the Asset Manager. However, if you have other files that needs to be hosted, you need to find other solutions. Dropbox is one. This is how you use Dropbox as a host for .js files to name one example: