From Cisco perspective we are not allowed to use image on GNS3. In the new GNS3 1.4, there is a way to run a second category of switches and routers. For older images, we use and maintain Dynamips an emulator dedicated to emulate some Cisco hardware. GNS3 offers multiple ways to emulate IOS. R+Svms are all vendor Routing and Switching images that are larger in size but can be imported into GNS3. IOS images are lower in cpu and memory so you can fit more of them in one topology. IOS images are usually run in Dynamips however R+Svms are housed in a virtual machine.
Cisco IOS images for Dynamips GNS3 offers multiple ways to emulate IOS.
