The Word "It" After a Preposition

[Bobbert]

I assume the following sentences are correct since I have seen them written:

Haga clic sobre/en él (el icono) y luego resalte el texto
Click on it (the icon) and then highlight the text

Haga clic sobre/en ella (la imagen) y luego resalte el texto
Click on it (the image) and then highlight the text

And I assume this sentence is also correct since I have seen it written:

No me opongo a ello
I’m not opposed to it

Why is “it” translated as “él” in the first sentence and “it” is translated as “ello” in the last sentence? Can “ello” be used in the first sentence and still be correct?

I think what I need is a grammar rule or explanation of when to use él/ella and when to use ello/ella after a preposition.