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As Reza has already described, yes you can.  However, do be aware that copying images onto flash will increase the CPU (not normally a problem, but you might see the CLI get very slow.  Often loading a new image requires removing the running image, first.  If that's needed, if for any reason the stack reloads during the copying process, the stack may not boot normally.  Also, after the stack is configured for the new software (again if old software had to be removed), any premature reload will boot into it.  If you do have sufficient flash space for both old and new, you can keep or set boot var pointing to the old code.

Using the bin IOS images saves some space on flash and the image copying goes a bit faster, especially if you can open multiple telnet sessions and copy to multiple stack members concurrently.

When Reza mentions using a USB, you can, but you can also do what he describes using a TFTP or FTP server.

Some IOS upgrades' first boot take much, much longer to boot (as they also upgrade firmware).