Choosing the best image to use with GNS3 depends on your purpose. If you simply wish to use GNS3 to practice Cisco IOS configuration for certification, then the best strategy is to use Cisco 7206 routers. If you need to use SVI (VLAN) interfaces, use 3725 routers with the NM-16ESW module installed.
If you wish to examine what features are available for a particular router, perhaps because you are prototyping a design, then you can often tell many of the features that are likely to be supported from the name of the image, or use the Cisco Feature Navigator to explore more specific options, including the DRAM required to run a particular image.
In the next chapter, you will get to explore GNS3 internal communications as you examine the many pieces that go together to make GNS3 and how they communicate with each other.