Many websites claim to offer "free" Cisco IOS downloads. Using these sources is as they are often illegal and pose significant security risks, as the images may be bundled with malware.
While IOS images are highly sought after, GNS3 supports many other vendor images and open-source alternatives that are often easier to obtain: where to download gns3 ios images
You can download free, legal images for VyOS , FRR , or OpenWrt which are excellent for learning routing protocols without licensing hurdles. Warning on Third-Party Sites Many websites claim to offer "free" Cisco IOS downloads
If you own a physical Cisco router (like a 3725 or 7200), you can legally copy the IOS image file from the device’s flash memory and import it into GNS3. Alternative Image Types for GNS3 legal images for VyOS