Often times the posters will upload the full-res photo. Summitpost just lowers the resolution for faster viewing. To see the original upload resolution just click on the picture.
You see the photo, it's made smaller to fit comments, etc. on the page
Then you just click on the photo and it shows you the full resolution image, which in this case is pretty big.
Do note, that they won't all be like this (e.g. quality, mp's) And make sure it's cool with the original poster that you use their photo. I, personally wouldn't care, and I don't think many people will, but it doesn't hurt to ask.
Hope this is what you're looking for,