Hi,
That URL information is probably stored somewhere internally in Envia Gallery, or in a meta field or something similar.
Returning the images by tags – do you mean searching them by those tags, or using those tags as filters? It all depends on how those tags are stored. If those are taxonomy terms, then it should be all right – by default the plugin does search those.
I am not familiar with the inner workings of Envia, so it all comes down on how that information is stored.