I believe it's because you're using a source like this:
i1.pixiv.net/img51/img/jyudo_fu/35756894_big_p10.jpg
When you should use:
http://i1.pixiv.net/img51/img/jyudo_fu/35756894_big_p10.jpg
If you don't use a full valid url, the site doesn't know to download from it instead of trying to use the file you give it (none, hence the "file does not exist").
