If the image size is less than 64MB, then booting from bootflash is OK. Beginning with Sup720 12.2(18)SXE and later versions, most Native IOS images exceed the 64MB capacity of all internal partitions, including the sup-bootflash: or MSFC3 bootflash:
partitions, so for 12.2(18)SXE and later an external flash device of at least 128MB is required with Cisco part number MEM-C6K-CPTFL128M=. For multiple concurrent image support in this boot device, the external 256MB CompactFlash Cisco part number of MEM-C6K-CPTFL256M= is required to use as the external boot device.
Some feature sets of versions > 12.2(18)SXE result in a smaller file, but as long as it is less than 64MB, there is no problem with loading it onto onboard flash.