Yes, as we discussed in another thread, this is 100% dependent on the viewer application you use. Images are zoomable in iBooks, but not in the ES Image Browser on the EVO. Nook and ADE handle it differently, as well.
In short, there is no single "magic" solution to guarantee that your images will be full-page viewable or zoomable on a particular platform. Some platforms don't support this functionality at all.
You need to identify which platform/software you're really interested in, and figure out what features it supports. Or, try to make a single ePub file that is generally compatible with multiple platforms (difficult!), and accept that in some cases it won't work.
Is this for your own personal use? Or are you trying to create ebooks for sale?