[SOLVED] Centring images with CSS?

The title sort of says it all, but this is really confusing me. I go into the the Style Images section and add:

As far as my web searching goes, this should essentially centre all the images affected, all the images in my posts/pages. Instead it only applies to images in sticky posts. Look here. If you look at the sign image in the sticky post you can see it's centred, but if you scroll down the page to the other image (Ctrl + F, paste in "February 3rd, 2012") it is not quite so. This is really beginning to bug me... Any help appreciated.

-Dapizz