I am all for including images of primary sources on profiles. Not only do they provide an easy way to check the claims that are made on the profile, they also inspire confidence and, in my opinion, help bringing a profile to life. There are some caveats however (and I'm not implying they apply to your work): these images should not replace a properly written biography, they should be properly cropped to the relevant part of the scan, and they should not be copyrighted (see my thoughts on that below).
About your concern about storage capacity: if every of the 20 million profiles on WT would have an image of 1 megabyte, that would require a storage capacity of 20 terabytes. With hard drives of 10 TB readily available for a few hundred dollars (and server solutions for the same capacity for a similar cost per month), I don't think this should be a concern. And by far the majority of profiles do not have images.
I'd like to broaden your question a bit (not only about Dutch archives) when it comes to copyright. I'm not a legal expert, and the situation is different in different countries (in the UK for instance, there is something like crown-copyright that applies), but in most cases I think copyright laws do not apply (certainly not for the original documents, and also not for the scans and pictures that are taken of them). The originality of the work, necessary for copyright, is in most cases not enough in the case of a simple scan. One might even argue that our work of selecting, cropping, interpreting and placing into context of these images does constitute original work.
Again: I'm not a legal expert, so I might have interpreted things wrongly. Also, I'm taking a respect for privacy and proper citation and acknowledgement of the source as a given.