I think that all pictures hosted on the forum appear as thumbnails until clicked - certainly mine do, and I've posted a heck of a lot of them over the years. It makes the thread easier to scroll through, particularly for people on a slow connection. If they want to see more detail, it's there at the click of a button.
I suspect, but have no proof to back it up, that the one that you see that are large when you first open the thread, are the ones hosted off forum. Sometimes people post them at such a high resolution that scroll bars are no longer on screen, so it's impossible to see what they are trying to show you
There are occasions when high resolutions are justifiable (such as showing a detailed circuit diagram with text) but normally 640x480 is what to aim for.