All images for the ASA 5525 can only be executed on actual 5525 hardware, or a software platform that provides near 100% emulation of the 5525's h/w. This will be true of all switch/router/firewall images that were not created specifically as virtual network functions. These images intended to run on actual h/w can include code that executes on embedded CPUs, NPUs, and FPGAs, while virtual network environments like CML (or GNS3) execute on h/w platforms that have only an x86 CPU to support the hosted VNFs.
To run the 5525 images on CML, Cisco would have to create a complete emulator of the 5525 h/w that executes on an x86 CPU and package it up with the 5525 image into a VM or container. In theory, this is possible, but in practice it is, well, impractical. There would be no ROI to Cisco in creating and maintaining commercial versions of h/w emulators for each of its hundreds (if not thousands) of switch/router/firewall h/w SKUs. Cisco does use emulators and simulators during the development process of ASICs, Platform Independent code features, and even complete routers, but there are not h/w emulators (written in s/w) for each and every product they produce. The emulators and simulators that do exist, going back 20+ years ago to IOU (IOS on Unix), are rarely exposed to the public as they are internal tools for dev engineers. One such h/w emulator that has been made available publicly is for the Cisco 8000 routers which, should be noted, requires substantial resources in the host system to run a single emulated router instance (32G RAM per instance recommended):
https://www.cisco.com/c/en/us/td/docs/iosxr/cisco8000-emulator/cisco8000-hardware-emulator-datasheet.html
I need to emphasize that h/w emulators are the exception and not the norm, as Platform Independent code can be developed and tested using virtual platforms (IOL, XRv9K, CSR1Kv, Cat8Kv, ASAv, etc) while Platform Dependent features are typically developed and tested on the actual hardware itself, as h/w emulators do not exist for every Cisco product and the PD code must be tested on actual h/w before it is released.
So where does this leave you? CML allows you to model the control-plane and data-plane (to a limited extent) of your network through the use of virtual images that approximate the functionality of your actual network elements. If your network runs in the cloud on Cisco virtual images, CML can support those same virtual images. If your network is h/w-based, then the virtual images you run will represent trade-offs between the rapid-prototyping capability of a s/w lab and the lack of 100% fidelity between the virtual images and your actual h/w elements.
TL;DR: Where can you find specific images to run on CML? Well, you can't, if you are looking for images that were created to run on h/w. What you can find on Software Central are the latest virtual images that can approximate the control and data plane behaviors of your h/w platforms.
Disclaimer: I am long in CSCO
