Is there a point (in terms of thumbnail size) at which you could safely (without breaking copyright) use a small version of other's pictures downloaded from the web?
No. Size has nothing to do with infringement. Only permission status.
And, in practice, whether you're actually going to publish the result.
Literally, it's an infringement if you take it off the web and use if for your own amusement, but in actuality, because it's unenforcable, you can do it with impunity.
Once you move to making a poster, or any other public airing of the composite, then you need to locate every copyright holder and arrange permission.
So have fun, and keep a database of the URLs of every image you have on hand.
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Emily L. Ferguson
mailto:elf@xxxxxxxx 508-563-6822
New England landscapes, wooden boats and races, press photography http://www.vsu.cape.com/~elf/
