As AP IOS is always distributed as a tar file, the AP cannot directly execute such a file (thus, if you were to copy c1240-k9w7-tar.124-25d.JA1.tar directly onto AP flash, and then try to boot it, this could not work.) The tar file contains, in addition to the IOS image proper, the radio firmware files, the HTML GUI files (if present), and various other files.
The AP IOS tar file must be unbundled into AP flash using the archive exec command (this is done in an automated fashion when a lightweight AP is upgraded after joining a WLC.)
After unbundling, the IOS image itself be in a file called flash:/platform-featureset-mx.version/platform-featureset-mx.version - for example, flash:/c1240-k9w7-mx.124-25d.JA1/c1240-k9w7-mx.124-25d.JA1. The AP is configured to boot this image if the bootloader BOOT environmental variable is set accordingly.
To see what IOS image the AP is configured to boot, examine the BOOT variable.