Its just a trck
<<< click this
The key thing is the size of the thumb.
Standard post layout gives you a /thumb640x480 at the end of the url. The thumbs are not generated on the fly, thats a lot of power, instead, they are generated when you upload and stored on the server.
For MOST thumbs we have these sizes:
You can choose from those - I have been working on adding another size, but thats not needed now we have the display from inline.
Those sizes in the post above are for the thumbs. The original sizes of 1000x800 are for the big pics - those are what you upload. You already have those....