I 'caught' the correct path where the executable and the HTML-file can be found in App.Path .
I want to open the OpenMe.htm file when I click on a picture, so inside the Picture_OnClick it looks a bit like the following:
ToOpen = Shell(App.Path & "ShellExecute.exe /F:App.Path & OpenMe.htm")
After "/F:" there has to be the path of the file to open, but as you can see it's wrong. Yet, when I use quotes after "/F:", it gives an error.