We have to take into account a fairly wide range of bandwidths, running from dial-up to big broadband. (Don't forget also that people's Internet access may temporarily change when travelling.) Then we need to remember that, nice as it is to have visuals, the blog layout favours text. The central column can be pushed wide by an overwide pic. PeWi points out another constraint with work firewalls. That should rule out thumbnails. 600 px fits into a 1024x768 display, so that seems reasonable. (There is no difference, from this point of view, between the diary and the comments.)
This seems unfair to fine photos that deserve to be seen full-screen, but Scoop isn't the right content management system for that. Even the Web pic servers place limits.
You don't need Photoshop to reduce pics. Irfanview is freeware and efficient. I'm quite willing to give explanations on how to use it.
My guess is that most people aren't, and didn't, but I'm more than willing to be proved wrong about that.
But I'll be surprised if it's a serious problem, because all we're doing is copying what sites like Flickr and Picasa and Photobucket do already - and they don't seem to have acquired a reputation for exclusivity.
If the thread loaded well for lower bandwidths, I expect it's because most people did in fact reduce the size of their JPEG files. There seemed to be a consensus about it - then some comments suggested it might not be worth bothering about, and the polloi could just use a collapsed thread. Which is what I reacted to.
So I figured people can discuss in the comments whether they have any problems with graphics and that will give a better gauge. Ad astra per aspera
