Yeah, so images are difficult. Please do not size them before uploading them.
When writing the wiki, I found that there is no point in having an image that is not full width. What would that be? Screenshots are full size. Logos and so on can also fill the whole screen. Consider that we do not know the screen size of the user. Having an image sized to half the width of the white column would maybe work on your desktop, but not on someones much smaller phone screen.
The only reasons I know of is visual appeal and faster page loads (for a smaller sized image). And agreed, screenshots are full size.
I don't find any captions. I must be looking in the wrong place or at the wrong image! (My wife says I am blind!)