Functionally and generically, they are the same thing for devices like routers and switches and firewalls ...
Some "images" consist of more than one binary file ... like code for specific blades or modules (like a radio module on an AP).
In most cases, it's just a naming convention and they are the same.
For some systems, like computers, a binary file is an executable ... it gets loaded into some flavor of memory, usually RAM, and the instructions are executed. During the initial load, some code is typically executed to initialize the various sub-systems. All of this initialization takes time (think about booting a desktop or laptop computer from power-on to "ready-to-go").
One way to speed things up a little is to save the initialized environment as an "image" or snapshot of the "ready-to-go" state of the device.
Once the snapshot is saved, it can be loaded directly into a block of memory without the need of all of the initialization code or interpretation of the binary code. This tends to make things "boot" faster. As long as the system's hardware configuration hasn't changed since the snapshot / image was taken, it should be exactly as if the device was never turned off. Some desktop and laptops have a "hibernate" mode .. everything is powered down, but it boots up as fast as the snapshot image can be pushed into the memory from the hard drive.
If you have a specific example, post it up.
Good Luck
Scott
