Do you mean the files on this page like "asa917-11-k8.bin" ? Just Google the filename...
To download directly from Cisco, you need an active service contract. I'd also prefer to source the file from Cisco rather than risk some dodgy image of dubious quality.
Cisco give you a filename.. and a checksum.
So having searched for asa917-11-k8.bin and downloaded it from the first source I found, the checksum is:
$ sha512sum asa917-11-k8.bin
bdc0b000e282457a1d3858aafffe5b1aead1c0ac1cad74eaea8d7b024f8398afde9cf9ded9ad74a1b5b3713a4e05cc109022ac685f6acc9ba8960b6aebff2c3f asa917-11-k8.bin
I'll let you do the same, and work out the odds of an SHA512 collision on a modified firmware image..