How do I upload an image into a comment?

My Little Pony - 1992 Edition
Wallet After Summer Sale -
Not a Llama - Happy April Fools Day!

fluffy sylveon
Surround the URL with exclamation marks, no spaces.
For example, type "!!" to get the following:

If the image isn't anywhere online, you have to upload it to an image hosting site. I recommend Imgur, you don't have to make an account and it only compresses images if they're so large you probably shouldn't be embedding them in your posts anyway.
Economist -

I don't pay my taxes
If the image is on derpibooru, then use the >>##t or >>###p format, which uses the image ID, which you can find in the URL. OR there is a section in the sharing tab you can copy from:

please do not use !URL! format for images that can be shown via the >>##p/t method
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Syntax quick reference: *bold* _italic_ [spoiler]hide text[/spoiler] @code@ +underline+ -strike- ^sup^ ~sub~