GNS3, compatible Cisco IOS images

You should be able to find these with Google:

1700
c1700-advipservicesk9-mz.124-15.T7.bin

2600
c2600-ipvoicek9-mz.124-4.T1.bin
c2600-telco-mz.124-2.T.bin
c2600-adventerprisek9-mz.124-18.bin

2691
c2691-advipservicesk9-mz.124-15.T6.bin
c2691-adventerprisek9_ivs-mz.124-9.T7.bin

3640
c3640-jk9o3s-mz.124-16a.bin
c3640-a3jk9s-mz.124-16a.bin

3660
c3660-jk9o3s-mz.124-15.T5.bin
c3660-ik9o3s-mz.124-15.T6.bin

3725
c3725-adventerprisek9-mz.124-15.T5.bin

3745
c3745-advipservicesk9-mz.124-15.T6.bin

7200
c7200-adventerprisek9-mz.124-15.T6.bin
c7200-adventerprisek9-mz.124-20.T.bin
c7200-advipservicesk9-mz.124-20.T.bin
c7200p-adventerprisek9-mz.124-20.T.bin
c7200p-ipbasek9-mz.124-4.XD9.bin

ASA8.1-1 ASDM6.1-1
PIX 8.0-3 ASDM 6.0-3 ASA 8.0-3
PIX 8.0-2 ASDM 6.0-2 ASA 8.0-2
PIX 7.2-3 ASDM 5.2-3 ASA 7.2-3