I'm not sure when this started happening but I was nearly at the end of the Plogger customization. You can see that the thumbs are floating left as they're supposed to in FF, Opera, Netscape... but in IE they're displaying in one vertical column.
I am far from being a css wizard but it does look like you may want to change the display: block; in the below rule to display: inline;
from w3cscools.com: block - The element will be displayed as a block-level element, with a line break before and after the element inline - The element will be displayed as an inline element, with no line break before or after the element