Update README.md

Author: flxholle

README.md

@@ -23,7 +23,7 @@ I added things like:
 You are welcome to contribute with issues, pull requests and ideas. And to contribute to this project, please fork the project and submit a pull request. 
 
 ### Screenshots
-<img alt="1 screenshot" src="screenshots/timetable_screenshot_1.jpg" width="31%" style="max-width:100%;"> &nbsp;&nbsp;&nbsp;&nbsp; <img alt="1 screenshot" src="screenshots/timetable_screenshot_2.jpg" width="31%" style="max-width:100%;"> &nbsp;&nbsp;&nbsp;&nbsp; <img alt="1 screenshot" src="screenshots/timetable_screenshot_3.jpg" width="31%" style="max-width:100%;">&nbsp;&nbsp;&nbsp;&nbsp; <img alt="1 screenshot" src="screenshots/timetable_screenshot_4.jpg" width="31%" style="max-width:100%;">&nbsp;&nbsp;&nbsp;&nbsp; <img alt="1 screenshot" src="screenshots/timetable_screenshot_5.jpg" width="31%" style="max-width:100%;">
+<img alt="1 screenshot" src="screenshots/timetable_screenshot_1.png" width="31%" style="max-width:100%;"> &nbsp;&nbsp;&nbsp;&nbsp; <img alt="1 screenshot" src="screenshots/timetable_screenshot_2.png" width="31%" style="max-width:100%;"> &nbsp;&nbsp;&nbsp;&nbsp; <img alt="1 screenshot" src="screenshots/timetable_screenshot_3.jpg" width="31%" style="max-width:100%;">&nbsp;&nbsp;&nbsp;&nbsp; <img alt="1 screenshot" src="screenshots/timetable_screenshot_4.jpg" width="31%" style="max-width:100%;">&nbsp;&nbsp;&nbsp;&nbsp; <img alt="1 screenshot" src="screenshots/timetable_screenshot_5.jpg" width="31%" style="max-width:100%;">
 
 ### Licence