So here we have two camps - ones (myself including) are content with medium sized pictures and others want images blowed up to size of screen. As screens aren't all same it's not easy task to design layout fitting all cases.
On 3:4 ratio display some pictures creep out of frame horizontally, on wide displays portrait oriented pictures may not fit vertically (honestly, I think I've seen images not fitting wide displays also horizontally!). Scrolling display to see whole image kills most beautyful pictures....Imagine sitting behind printed book and unfolding each page to see missing 1/4th of image....
Also keep in mind left part of screen is dedicated for user info, vertically some part is taken up by browser (some more, some less, and how you configure 'em but there are different visible areas) and thus not not whole screen is reserved for picture.