GNS3 is one of the most prominent software for performing the networking LABS. if you want to download the IOS images then you can go through the below-given links.
GNS3 is far better than the cisco packet tracer because in GNS3 you will get the real flavor of labs where you can capture the packet and also if you want you can connect your GNS3 router to the internet.
Note:-GNS3 uses your computer’s physical RAM and processor so make sure you have sufficient RAM and a multi-core processor.
Another thing that is very important in GNS3 is idle-PC, by default the GNS3 calculates the idle-pc value but I recommend you to try with multiple idle-PC values which are very less memory and processor intensive.
The above-listed images are working perfectly in gns3 and you can use them for your cisco lab.