The size is chosen by the person when they upload it. It defaults to quite small, so you have to remember to set it larger when you upload pictures. You can enlarge any web page by typing control + but then the picture just gets blurred, control - to make the page back to normal size.
Jane I have just copied this from Daisydoos original "name this tune" thread. It's not so much a question of making the picture bigger but not shrinking it, or rather letting the system shrink it down when you upload it.
I think there is an automatic "resize function" which wants to make the image smaller to save space when its being uploaded.
I have posted what I can see as a transcript on the original thread.