Hi Gilles,

Thanks for your email!

Since I'm new to the Wikimedia ecosystem, I have many questions on how to do that:
- first of all, is it acceptable to associate one (or several) external links to a file/picture stored on Commons?

Depends, but generally, yes. We typically link back to sources (eg Flickr), to authoritative sources (museum database, cultural heritage database, biodiversity catalogue, etc.)
 
- is there any existing format to point an OpenStreetMap resource? Should I create a template for that? and/or use other_fields in the Information template?

We do have <https://commons.wikimedia.org/wiki/Template:On_OSM> but I have never been happy with it. I think it would be worth creating one. What wouls this template do exatcly − just create a link to a OSM node?
 
- apart Title and Description, is there any required or strongly encouraged fields that the user should enter?

Date should be automatically filled by the upload process.

Author is mandatory, as well as a license.
 
Another request, a little embarassing... Yesterday, I've tried to send a dummy picture to Commons [5], just to see how it works, but now I'm not sure how to undo this... I've found the {{speedydelete}} tag, is it the way to go?

Yes, it is. I went ahead and deleted your test upload. That’s no problem.
 
By the way, is there any dev instance of Commons where I can play safely with the API?
Yes, there are several ; I believe <http://test.wikipedia.org> is your best bet (please someone corrects me if I’m wrong)
 
Hope that helps,
--
Jean-Frédéric