@chinatsu Will a twitter profile link work? Or do I need to link to the image the artist uploaded to Twitter itself?
Well it depends on the source. For images from a tweet you should link to the page of the tweet, not their profile and not the direct image. For images from sites like Pixiv, the direct image URL should be used.
It's all explained very extensively in the wiki in entries like help:image source. If you read the articles you see every time you upload it should be pretty clear what you need to do. Was there anything in them that wasn't exactly clear?