Time to shake the nest...
I'm currently redeveloping my blog. Nobody but me knows the URL. I have Google ads on the site already for testing. 99 out of 100, I'll get an image ad from Google, rather than text. That would appear to suggest that traffic numbers have no impact on whether you see image or text ads from Google.
dotcomUnderground seems to have a good idea of how Google determines whether to show you images or text. I'm not sure where that leaves my test/new blog, but I just gathered I would throw it out there to confuse you all.
FWIW, and I'm sure you know this already, you can easily choose to turn off images ads or text ads, and just display one or the other. This is done during the setup process for a new Google advert.