Your images appear cut off on Reddit if you post an incorrect Imgur link.
The link you want to post should look like https://i.imgur.com/random.png (image link) instead of https://imgur.com/a/random (gallery link).
Don't use any of the buttons Imgur has for getting the link. They all give you a link to the gallery, which doesn't show a large thumbnail on Reddit.
To get the correct link, right-click on the uploaded image and copy the image link/address.
To avoid this altogether, upload your images directly to the website, either in the Post Scheduler or the Content Manager. It's also faster.