fix: use hash-based image identifier to prevent caching, use colored background for icon fallback

[daniel-mader]

Description of change

• use hash-based image identifier to prevent caching
• use colored background for icon fallback

Links to any relevant issues

• fix: use hash-based image identifier to prevent caching, use colored background for icon fallback #396 ("dirty" merge)
• fix: use hash-based image identifier to prevent caching, use colored background for icon fallback #415 (closed in favor of this PR)

How the change has been tested

Successfully running in v0.7.3

Definition of Done checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[maiertech]

I suggest to add 2 comments.