Should I Use “these” or “those” When I Refer to Something in the First Sentence?

Solution 1:

Neither sounds good, but "those" is more natural.

You are asking about which demonstrative pronouns (this, that, these, those) most appropriately mean "the aforementioned." It's hard to specify a rule that answers the question generally. But I would say you only use "these" when you want to emphasize that you mean specifically the aforementioned photos, and NOT some other ones. In other cases, default to those.

I know you said that changing the text was not an option, but it sounds terrible as it is. Just for the record, I would rewrite it as:

I’ve posted 5 photos on my timeline. Have you seen them? You’ll love them! They were taken in France.

When it's phrased like this, the these/those confusion is irrelevant. Also, reusing the word "photos" multiple times, as is done in the original, makes the text feel stilted and robotic.