Axel-
First, use the extensive public domain archives
Pictures are public domain if they
1) have been created before 1923
2) are produced by the US government.
Obviously that leaves a lot of essential material, especially given that
Wikipedia is not a US-only project, and most other nations don't have an
equivalent public domain rule.
Next, once our foundation has
money, we can try to acquire the copyright of selected important images
we need and cannot get in any other way.
The problem with that approach is that we need an exclusive world-wide
license, since we need to allow unlimited redistribution and modification.
This is *very* expensive for most professional photographs, and impossible
for many.
Regards,
Erik