protip, you can copy the image from Gelbooru and then hit CTRL + V when you're focused on the URL field, this will upload the image to lemmy if it's less than 1 MB.

Otherwise you can upload the image to a file host like catbox and copy the URL it gives.

Lastly you can use the image url from gelbooru and copy that; but they don't like their CDN to be used as a file host for other services as they don't generate revenue.