Click 'edit' on your original post, then put your image's URL into the "URL" space and save.

There's actually no difference between using an external image host and uploading to lemm.ee in terms of end result - all image posts on Lemmy are technically just URLs that are filled out.

Sorry I just saw how you posted it - you posted the link to the image's webpage but not the direct link to the image itself. You'll need to right click the image and e.g. "Open image in new tab" or "Copy image location" or something so that you end up with a link that actually ends with your image file, like this:

https://images2.imgbox.com/4f/6c/yf5t7SiE_o.jpeg

catbox.moe doesn't do the image page thing and will just give you a direct URL, if you find that easier.

Depending on what you are using the web ui, or an app should parse the image from the link. You can try using https://squoosh.app/ and really shrink down your image without real quality loss.