If you wants to perform any router based on lab exercise in GNS3, first, you need to download ISO images for routers. After downloading IOS images, you need to add router IOS image in GNS3

• In the GNS3 console, click Edit and select Preferences to open the Preferences window. Click Next to add a new router IOS image in GNS3
• In the IOS image page, select the New Image radio button, click Browse and select the router IOS image that you want to add in GNS3
• In the Name and platform window, specify the name and platform for the IOS image and then click Next
• In the Memory page, set the desired Default RAM size and then click Next
• In the Network adapters page, ensure that slot 0 is already listed and selected. Select the additional slots, if required, and then click Next
• In the WIC modules page, select the WIC modules to add additional modules as per your requirement, and then click Next
• On the Idle-pc page, click Finish and then click OK to close the Preferences window.
• In the GNS3 console, you can see that router is added. Drag it to the work view area. Right-click on the added router and then select Start to start it.
• Open the console of the router and use it to perform the hands-on lab exercise in GNS3.

That’s all you need to do to add router IOS image in GNS3