First got into stock photography as a consumer of images, mostly for church, and then started submitting more of my own. Used to have a gallery on freeimages (SXC), so some shots have been available elsewhere in the past - but new stuff is RGBStock exclusive.
To answer your more specific question, I suspect in uploading to sxc.hu (or, for that matter, to RGBstock) we grant some right for the owner of the site to distribute the image. When a site is bought, that granted right to distribute will be passed on. Im presuming that our permission for the site owner to distribute our images, as granted originally to sxc.hu, is still valid/extant... There may have been some communication around the time the site was bought - but I suspect the buyer would *not* have wanted a whole bunch of the value of the site (the image collection) disappearing if they went and explicitly asked for fresh permission when they didnt need to...
Incidentally, Jazza, try visiting www.sxc.hu and see where you end up - there is definitely a connection!!
If you kept your sxc.hu portfolio online there for a while after the site was bought (ie didnt delete it) then I recall images were then migrated to what is now freeimages.com. I did this for a while, then actively decided to remove them from freeimages and deleted them - had a bit of a back and forward because the default when you de-activated your portfolio was that theyd keep a copy of your images on their servers, but not make them publicly available... I objected and asked them to remove the content from their servers completely. My images were still on freeimages in 2015 or so - I cant track down exactly when I asked for them to be removed.