I am having the same issue (and in looking at this example db link from the About page of this project: http://www.brandlehner.at/Brandlehner/c2picture.nsf/albums/CBRR-67LSLH, it looks like the same issue is popping up). Here is the formula for selecting an image in the sub-album form: "Photo Album_sub_en - Form" : @DbLookup("Notes":"NoCache";"":"";"LookupsAllPicturesbyAlbum";AlbumName;3;[FailSilent])
In looking at the view "LooksupsAllPicturesbyAlbum", I see that the field being looked up is "$$Categories". Further investigation from the used to import pictures "ImportProfile_en - Form" shows that this field gets populated with the Album name, not the Sub_Album name. Therefore, if you do not have any pictures saved under the main album, you won't see any choices for pictures in the Sub Album form. I changed the first field in the "LookupsAllPicturesbyAlbum" view to the following formula: @If(album_sub = "";$$Categories;album_sub). I don't like having a formula like this in here for the selection criteria, but it was a quick fix ;-). Haven't tested all of the other functionality in this application yet to verify I haven't broken anything else with this, so make sure you comment your changes if you try this.