Update:
Turns out that its +*even simpler*+, or should I say *more general*:
Basically: +*Drag the folder you wish to de-quarantine to almost
any program in the Dock* (or in the Finder)+.
If it makes the program react in any way (for example starting), then the content of the dragged folder has most likely been recursively de-quarantined, seemingly regardless of file types of the content (including applications).
If you only want to de-quarantine, but do not want to open the folder's content in an application, you can for example alt-cmd-drag the folder to TextEdit, which will inform you that it doesn't open folders, but the files will nevertheless be de-quarantined.
Or you can most simply type *open <*+and drag the folder here+
> in the Terminal!
As I wrote before, the above is for some reason dependent on the dragged folder itself having the com.apple.quarantine attribute, but that will always be the case if it was extracted by +Archive Utility+, which I believe is the major source of the need for de-quaratination in the first place. Which is why using StuffIt Expander for extracting picture archives is another good workaround.
