It depends in part on whether your usage is editorial or commercial. While credit is mandatory for editorial usage, it is not for commercial. So if you're using the images for commercial purposes, you're free to credit the contributors and DT however you like or even not at all (although it really is greatly appreciated).
However, based on DT's guidance, crediting both the contributor and DT for editorially used images is necessary, although I can't swear that it's necessary to use the suggested format.
Even thou it is not required, I would recommend crediting authors in commercial use as well if possible (especially online use) to strengthen protection against illegal use (-for example steeling legally licensed image from somebody's website looks more risky for pirate, if the copyright is declared on the page).
However if I am using say a DT silhouette, and then collaging a pattern from a DT photo -- do I credit both contributors? Or since I created this new image for use (article layout) then is it necessary?
I might be wrong, but I perceive it like having three architects designing the house - 1st one designed the basement and first floor of the house, - 2nd architect designed the roof, attic and interior of the house - and than comes the 3rd architect who designs the landscaping, garden and exterior of house... Who should be listed as author of the property design?
From this angle of view you have to list all three authors as owners of copyright... and that copyright of final image is shared by 1/3 by each of involved authors... (so no licensing of this file without permission of all three) But that is just my understanding and I am not a lawyer - but it makes sense to me this way...