Add explanation about forged image

Lab03 - Let's Buy Some Juice/README.md

@@ -26,3 +26,8 @@ Result:
  2. Type `<iframe src="javascript:alert('hello')">`
 
 <!--- ToDo: Insert Picture -->
+
+## Forged Picture
+This challenge is impossible due to the `/profile` page only reading the username from POST variables.
+
+POST variables cannot be set through links and thus can't be set in an `img`-tag's `src` either.