Finally it works. Thank you so much for the insight about case sensitivity.
Code:
Array.IndexOf(myArray, UCase(strFileName)) + 1
renders the right index of the current image. As a parenthesis I wonder why the usage of this code requires me to remove "+ 1"? It works like a charm, but why does it render the wrong value when I leave "+ 1" there? It doesn't render the wrong value if I just want to present the index on the screen...