again I think by looking at this the problem is what I suspected the first time... the fact that pre-defined display images are created from your original file.
Notice the header height between your firefox and IE browsers... the firefox header takes up less vertical room... not much less, but less none the same.
In the second example where it appears to be working, because the thumbs are on the side, you have more vertical real estate. notice the images are the same size, but because of the thinner header, the firefox browser shows the image a bit higher and should have just slightly more room to go lower... there is space there below it, but not much.
in the first example, where the thumbs are on the top, you have less vertical real estate to work with, and apparently the difference in size of real estate is just enough that the IE image has to switch to the next smaller size 'display image', thereby leaving more room at the bottom.
To illustrate this effect further, you could... what's it called? not maximize your browser on your screen, then while the image is being displayed, slowly drag the bottom up to change the vertical real estate. you'll notice that instead of the picture resizing consistently with the window, it will suddenly jump to a smaller image to fit within the window, and stay that size until the window shrinks small enough that it requires a size change again.
That's the trade-off in the way they've designed it. faster display because the display images are created and stored when you upload your originals and don't have to resize on the fly, but depending on how much available space you have, it could show more space below the image...
I know there had been discussions about this in the past, and some things suggested were more display sizes (more sizes in between the current set) to minimize this effect, and others suggested just going to the 'resize on the fly' method, giving up some speed to maximize display all the time. I do not know where anything is in the process on anything however.
Hope that explanation helps show why it is happening anyways...
~Kevin
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