Image CC-BY attributions missing, MD5 for filenames

[adamhrv]

FYI images redistributed from YFCC100M require attribution of the original author's work (licenses are available in the original YFCC100M dataset file).

The YFCC100M authors also recommend using their MD5 hash as the identifiers instead of arbitrary sequential numbering. This will make the CC license lookup easier too.

if you wish to release data that refers to photos/videos from our dataset, please refer to their hashes instead of their identifiers.