How many Unique hits per day should I be getting before I should start selling links / banners on an image host?
LOTS. The traffic is usually crap, so most advertisers won't be interested unless you have alot of it. However you could throw up some contextual advertising before then and see how it performs for you (i.e. Adsense). Maybe you'll get a decent click-through-rate and make some money as the site gets more and more popular.
Selling links on your website is not all about the traffic your website has it is usually a mixture of the traffic your website has plus the page rank your website currently ranks at. I would suggest if your primary source of income you want is through link sales, that you concentrate on both increasing your page rank and traffic to your site.
What about checking up a CPM network for now? Places like Casale Media, Media Fast, etc. They'll pay you a flat rate per CPM, so as your site grows so will the revenue. Then once you've got say 5-10k uniques/day start looking for advertising directly with other people as you'll actually have something they'd be interested in.