Update README.md

Author: luizbizzio

README.md

@@ -45,10 +45,10 @@ The installer downloads the release files, creates the local app folder, adds sh
 <table align="center">
   <tr>
     <td align="center" valign="bottom">
-      <img src="assets/demo/tray_menu_detailed.jpg" alt="Tailscale Control detailed tray menu" width="420">
+      <img src="assets/demo/tray_menu_detailed.png" alt="Tailscale Control detailed tray menu" width="420">
     </td>
     <td align="center" valign="bottom">
-      <img src="assets/demo/tray_menu_compact.jpg" alt="Tailscale Control compact tray menu" width="420">
+      <img src="assets/demo/tray_menu_compact.png" alt="Tailscale Control compact tray menu" width="420">
     </td>
   </tr>
   <tr>