As an alternative to Scotland141's perfectly valid answer, you can use of which, although in this case the word order is slightly different:

An image is read and sent to the function, the output of which is displayed.

As an alternative to Scotland141's perfectly valid answer, you can use of which, although in this case the word order is slightly different:

An image is read and sent to the function, the output of which is displayed.

You could also maintain something closer to the original word order (sounding more formal):

An image is read and sent to the function, of which the output is displayed.