Discovered why the links to postimg.org are broken. An undated bulletin on the postimages.org/contact page says:

postimg.ORG domain is locked by Registry, no prior notice.

While we hope to resolve the issue, we chose postimg.CC as our new home.
Please update codes embedded in your websites.

Simply replacing postimg.org with postimg.cc in the URL restores the link to the image file. I guess the question remains whether to await a (hopefully) timely restoration of the postimg.org domain to postimages.org or to plough ahead finding and editing one's old posts.

Pity that Wikispaces has no global search & replace. Therefore, more speedily done on the set of exported pages or on the migration host, unless you plan to migrate the image files from postimg.org in which case the difference in efficiencies may be minor.