Vertically centering is a big dilemma and requires display: table-cell; Rule but some browser doesn't support it (IE),
the position:absolute; way also doesn't work, since we don't know what the thumbnail's height value. (Everybody uses different values for thumbnails.
So all in all, there is nothing i can offer you now to make the pictures to be centered perfectly in all situation. But you can use the "padding-top:" Rule to give some space to the top of thumbnails. So they won't look as they stick to the top.
For this purpose, open the FlexibleVM2Global.css file, which is right here:
add "padding-top" rule to it and don't forget to reduce the max-height: value depends on how big padding value you are going to type, since the padding value will be disregarded while calculating the max-height of thumbnail, it should eventually look like this: