Minor document update with clear images

[srbala]

Document contains images not visible to read so, added new images. On step 3, check label change over the time, fix the label

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your master branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [x] Link to relevant issues in GitHub or Jira - Plugin document site https://plugins.jenkins.io/github/
• [x] Link to relevant pull requests, esp. upstream and downstream changes - N/A
• [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue - N/A

[srbala]

Would be nice to retain the original image file names. Anyway, it is a nice improvement, thanks!

@oleg-nenashev Can I replace the old files? OR old name with suffix?