This isn’t directly possible. You can add an image to the body of the post using the ![alt text](https://example.com/image.png)
formatting. Lemmy automatically chooses an image to display on link posts. You can’t override this on Lemmy 0.18.4, which Beehaw uses (I believe it’s possible on later Lemmy versions, but the official UI doesn’t support it on those versions).
Mlem dev here. You can attach a link to the post directly by clicking the link button in the post editor toolbar then tapping “Paste” (with a link copied to clipboard). We’re aware that this isn’t the most obvious UI, and are improving the design in the next version.