Revert "Remove missing image in README.md"

This reverts commit 1f8170c.

Author: microbit-grace

README.md

@@ -26,6 +26,8 @@ npm install @microbit/microbit-universal-hex
 
 ## Web Tool
 
+<img width="30%" alt="microbit-universal-hex logo" src="docs/img/web-tool-screenshot.png" align="left">
+
 An implementation example can be found in the
 [docs/examples/webtool.html](docs/examples/webtool.html) file. This web tool can
 generate a Universal Hex from two Intel Hex files.