The web is inherently flexible. Embrace the medium, don't impose a false limitation because it seems easier at first.
What do you define as 'mobile'? Will that change in 6 or 12 months?
There are more devices than the iPhone. Flexible sites just work* on almost every device including the iPhone.
There many eloquent and well-informed opinions on the subject:
- http://alistapart.com/article/dao
- http://blog.andyhume.net/responsive-by-default/
- http://adactio.com/journal/6120/
- http://futurefriend.ly/
There's no harm in ruling that all images created for the site are of a specified size, although scaling and resampling them to fit their allotted container is relatively heavy work. As with anything online within the browser - a medium designed to compromise - it's a balance.