Im not a lawyer, so all the stuff below should be taken with all the usual caveats – the main caveat is…whatever happens, it ain’t my fault :P
According to a EU Directive, the duration of copyright is now the life of the author, plus 70 years (it was previously 50 years).
You can read the thing here in two versions applicable to Belgium:
http://tinyurl.com/3ex7ed
http://tinyurl.com/9pvne
I suppose it means the images from 1942 are presumably covered by copyright (if the artist died in 1942 copyright wouldn’t expire till 2012…) and if the 1904 authors died of old age, conceivably those are still copyrighted too.
The authors estate might have passed ownership onto the institute, or on the other hand, the authors might be dead, but if they have surviving family…they might claim against you.
On a practical and realistic scale, I’d say if you’ve made an effort to track the copyright holder/s (by doing a bit of research) and credit the original author/s, then if anyone comes out of the woodwork, at least they couldn’t claim you hadn’t made an effort to be honest. At that point you could come to a deal.
I don’t really think it would get legal. You could even argue you’d discovered the images for the inheritors – I mean, if the inheritors knew the images existed, the institute would have records…right?
It also depends on what the nature of your project is. If its non-commercial it might be covered under Fair Use terms anyway, so you’d be alright.
On balance, due to the age of the images and if the authors are virtually unknown (and have not claimed active ownership for what looks like decades), chances are you should be OK.
I’d make an attempt to source more info on the authors though if you can, as a safeguard.
Are the images Belgian? If they’re from another country, (like the US) then different laws might apply.