In response to 1.
Its a hack to do it in document-links imo, document-links require at least one DocumentCategory and none of the categories fit pictures.
Saying that, its also the only option where you can validly hack it into IATI a bit without using namespaces (which I'll never use). We implemented it on aiddata.rvo.nl since it does enrich the context of the project, especially for their type of projects.
(there's more on the media tabs).
We used document category A12 (= Activity web page... well its at least shown on the web page!). Labelling + captioning would have to be merged in the title, we did not use it.