Add image headers for main readme and plugins

[Dabolus]

Before you submit a pull request, please make sure you have read and understood the contribution guidelines and the code of conduct.

This pull request is related to:

• [ ] A bug
• [ ] A new feature
• [x] Documentation
• [ ] Other (please specify)

I have checked the following:

• [x] I have read and understood the contribution guidelines and the code of conduct;
• [ ] I have added new tests (for bug fixes/features);
• [ ] I have added/updated the documentation (for bug fixes / features).

Description: Added image headers for the main readme and for each currently existing plugin.

Related issues: None