Assuming the method for selecting pics is the same as on LJ, they're selected by looking at the most used in posts over the last X months (I forget the period), so if you've imported then it'll select the icons used totally, and recent uploads are less likely to be selected.

It's not an ideal system, but it works.

FWIW, I'd also like to be able to chose my icons, it's annoying seeing some of my favourite "use in comments only" pics greyed out when some of the others are only ever used for occasional posts on specific subjects.