Tumblr images carry one of multiple available file name suffixes:
_75sq (sample; thumbnail)
_100 (sample; thumbnail)
_250 (sample)
_400 (sample)
_500 (sample; sometimes original, see forum #132817)
_500h (sample; height-bounded)
_540 (sample)
_640 (sample)
_1280 (sample; sometimes original)
_raw (defunct)
It is Danbooru policy to either replace samples or delete them. See image sample for more information. See help:tumblr on how to upload from Tumblr correctly.
