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).
If the pictures are for the UK, you can use the following site; http://www.geograph.org.uk/ You just have to write something saying who has the copyright, it'll explain it on the website. Dan gave me the heads up on this and we used a few on our Grampians page.
Generally if I use a photo from someone on SP, I just put a link to their user profile page under the photo on the page I'm creating. Haven't had any complaints about doing this...well not yet anyway
If you're afraid an SP user may go away and leave a hole in your report, consider asking the person if you can copy the pic and upload it to your profile, and make sure to give full credit to the person in the caption. Unless the member is a professional photographer or concerned you have other reasons to use the picture, I think most people would be okay with it.
I really don't think the chance of the member leaving should be that much of a concern, though, that it becomes an impediment to writing your report.
"Alcohol is like love. The first kiss is magic, the second is intimate, the third is routine. After that you take the girl's clothes off."
--Terry Lennox, The Long Goodbye (Raymond Chandler)
What I've done for multiple mountain pages for non SP photos was to contact authors whether it be on flickr, nwhikers, smugmug which most of them say yes and a few of them say no. I use html for images and often try and find a thumbnail too so that it does not slow down page loading.