How to get the id of the linked asset from a field (Content Delivery .net)

Still quite new to Contentful and I’m trying to read out a news item having an image field (being a link type field pointing to an asset).
So far, regardless of the model I use to read out the entry, the value for the image field remains null.
I do get the asset in the IncludedAssets property of the GetEntries reponse, but no idea how to know how to get a reference from the field value to this asset.
Please help…
Thanks, Maarten