Thanks for explaining! The feature you're trying to make work doesn't allow for uploading or downloading images (uploading is when you try to get an image from your computer onto a site, downloading is when you're trying to get an image from a site to your computer). It's for when you have images uploaded to a different website, then you can copy and paste the URL into the blank space. That will make the image show in your posts.
See how https://imgur.com/lXLOvyt does not end in .jpg or .png or .gif ? If you go to that link and right-click on the image, then click 'Copy image address' (or if your device is not in English, a similar phrase), it should copy a link that looks like this: