So - I think that yes, you could do this. You'll need to use a concat formula - concat(fieldwithURL, "/", fieldwithpicturevalue) for example, and use that for the default value of a field that is bound to a linked pic. Simple sample attached. Save the file locally, right click, select design & preview. You'll see in my sample the equivalent to your @picture includes a bit more of the path, that's just because I was looking for publically available images with similar URLs to kind of mock up the concept. :)