Last modified: 2014-09-05 12:37:52 UTC
The data currently returned by the imageinfo API assigns the author to "artist" and the source to "credit". Both of these are incorrectly labeled. In particular these labels make it difficult to figure out the proper attribution for the image, as it sounds like you should use "credit", but you should actually use "artist". https://en.m.wikipedia.org/w/api.php?action=query&format=json&prop=imageinfo&titles=File%3ALiber_Eliensis%2EPNG&iiprop=url%7Cextmetadata
Not an API bug, it's just returning what deeper layers of MediaWiki give it. It sounds like you want the CommonsMetadata extension.
See bug 57189 comment 1 for the rationale behind the field names. In hindsight, I agree it was the wrong decision, but I am not sure it is worth changing all the field names now - the API has other serious limitations and will be eventually superseded by the structured data API when wikibase will be introduced to Commons.