The two words people hear wrong
"Royalty" is a recurring payment owed each time a work is used. "Royalty-free" means only that this recurring payment does not exist: you settle up once and subsequent uses cost nothing extra. It is a statement about payment structure, not about price and not about permission.
Which is why the term misleads almost everyone who meets it. Royalty-free assets are frequently not free — many are expensive. They are almost never exclusive, so a competitor can licence the same photograph on the same terms tomorrow. And they arrive with limits: print runs, user counts, territories, and near-universal bans on reselling the asset as-is or using it in a logo or trademark.
Free and royalty-free are unrelated
A £400 image can be royalty-free. A £0 image can carry per-use royalties. The two words describe different things, and the resemblance between "free" and "royalty-free" has probably caused more licensing trouble than any other phrase in the field. Read the licence, not the label.