Cisco IOS/XE/XR/NXOS images are not FOSS (Free and Open Source Software); you must purchase Right To Use (RTU) licenses to run them. With an RTU license purchase, you receive entitlement to download the images. Cisco images can only be legally distributed to the public by Cisco, so any third-party site offering Cisco images to the public is violating Cisco's copyrights with pirated software.
The least expensive way obtain valid licenses is via a CML subscription. Note that the images distributed with CML are licensed only to run within CML, not EVE-NG nor GNS3 nor any other environment. If you want Cisco images to run on EVE, then you would need to purchase their licenses individually, which would far exceed the CML subscription cost.
There are FOSS NOSes (Network Operating Systems) available which can be freely (and legally) downloaded to run on EVE-NG or GNS3 such as SONiC or DANOS. While they will help you learn networking and virtualization technologies, they are unlikely to help much with Cisco-specific exam questions.
Disclaimer: I am long in CSCO
