The Word "It" After a Preposition
 

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.

The neuter pronoun ello can represent an abstract idea, thought, or event (something intangible).
The masculine pronoun él can be substituted for a masculine object.
The feminine pronoun ella can represent a feminine object.

None of these pronouns are interchangeable.

Thanks for the explanations, Rusty.

Using those explanations, am I on the right track with these sentences?

I don't want to read that book. It's not because I'm opposed to it (the book); it's because it has a sad ending.
No quiero leer ese libro. No es porque me opongo a él; es porque tiene un final triste.

I don't like her going out with Carlos. I don't think it's a good idea, but I'm not completely opposed to it (going out with Carlos).
No me gusta que salga con Carlos. No creo que sea una buena idea, pero no me opongo completamente a ello.

Yes, with the parenthetical aid you provided, you have it right.

I agree with Rusty.
Just a side comment: if you say "no me opongo a él", referring to a book sounds strange, because an object can't really be an adversary to you. But grammatically, the sentence is right. ;)

Thank you, Rusty and AngelicaDeAlquezar. That clears it up for me.