Learn Hardcore Norwegian: Jeg ser på bildene.

Because ser på is a phrasal verb meaning “to look at.” Without på, ser simply means “to see” or “to watch,” not necessarily “to direct your gaze at” something.

What does bildene mean and why not just bilder?

bildene is the definite plural form of bilde (a neuter noun), so it means the pictures. If you say bilder, you mean pictures in general (indefinite plural).

How do I form the definite plural of a neuter noun like bilde?

The pattern for many neuter nouns is:
• Indefinite singular: et bilde (“a picture”)
• Indefinite plural: bilder (“pictures”)
• Definite plural: bildene (“the pictures”) – formed by adding -ne to the indefinite plural.

Could I say Jeg ser bildene instead of Jeg ser på bildene?

Yes, but it changes the nuance.
• Jeg ser bildene means “I see the pictures” (you notice or recognize them).
• Jeg ser på bildene means “I’m looking at the pictures” (you’re actively examining them).