When you are reading a text post/comment your request goes from:

Your Device --> eviltoast.org (your instance) send you the text --> You receive [eviltoast.org]'s copy of the text

When you see an image that goes from

Your Device --> eviltoast.org gives you the link to image --> Your Device receives the link, then sends a request to retreive image --> OP’s instance --> OP’s instance send back the image, and you see the image

Most instances do not keep a local copy of the image. I don’t think I know any instance that makes a local copy. Your instance certainly does not.