Is there a way to flash a boot.img from recovery without a computer? Is there a way to flash a boot.img on your SD card without having to connect to a PC?

Is there an equivalent of the fastboot command?

I'm on a HTC Amaze 4G currently.
Once you have S-OFF you should be able to flash a custom recovery.  Just flash one that supports IMG files, such as ClockworkMod (ROM Manager).