Icons in the Debian menu system should be in XPM format.
While other image types (e.g. png images) appears to "just work", window managers are not "required to support them". Accordingly using non-XPM icons could break interoperability.
For more information please consult:
The tag is present in Lintian version
That is the most recent version we know about.
We use semantic versions.
The patch number is a commit step indicator relative to the
release tag in our Git
You can find the detection logic for this version at commit ffc17eb. For merge requests, please use the latest version in the Lintian check menu-format.