In principle, unless stated otherwise, any picture on the web is copyrighted unless stated otherwise, and therefore cannot be reused freely. I know, many people don't follow this rule, and usually it's harmless. However, publishing copyrighted material on Summitpost is not allowed unless you have permission to do so.
That said, there are still plenty of pictures on the web that you can
reuse, provided you credit the original owner or licence holder. Wikipedia is a good source for such pictures. For example, see this one of the Snowdon Massif from Glyder Fawr, taken by Chris Dixon on 7 September 2004:
Click on the image to jump to the wikipedia page where I found it. The page describes the conditions under which it can be used. Unfortunately, as I'm no lawyer, I have to admit that this legalese is a bit above my head, but I think it means that to use this image, you have to give credits to the owner (Chris Dixon in this case) and maybe you also have to specify where you got it from (Wikipedia, and I even gave the link).
Good luck on your TR,