I see they allow 8 different sized images. Do any of you know which Adsense unit sizes are the most commonly used?
Go look around at the sites they have listed that you think your ad would relate to, and it will show you what sizes those sites allow.
What if I was promoting something like ringtones or dating(Not that I would)? That could be a shit ton of sites to look up.
how many days does it usually take to start accruing a good amount of impressions?
day 2 of my image campaign running = 450 impressions as of now. at .40 CPC bid, i figure this will soon pick up.
do impressions usually pick up around day 4 or 5?
Are you running only one image size or are you running multiple sizes? If you're just running one size then your ad will only appear on pages/sites that have that size of Adsense unit available. That could be why you're getting a lower number of impressions.
I've got the three most popular image sizes running. However, I only have about 40 keywords, so I figure that's my problem. I am going to bump that up to quite a bit more and see what happens.
Content network can be very useful in generating quality traffic, but be sure to be able to track clickfraud so you can get refunded for crap traffic.