I will be interested to know more too as I have not seen any good open source tool in this space .
But here are few options that I know .
Monitoring on Container Images can be divided into two areas -
1 – Scanning /Auditing Images before it gets deployed to Kubernetes or any other container orchestration tool .
And , in this area there are tools like JFrog Artifactory X-Ray and some Sonatype Lifecycle tools and you can integrate them with your CI Pipelines .
2 – After an image is deployed - checking/auditing/scanning Image-Version/Checksum etc. ,in this space tools like Sysdig and Aquasec are helpful .
These tools provide lots of functionality , control and security for your Kubernetes Infrastructure .
All of these tools have open source versions as well .