Learn German: Hast du das Foto gesehen?

In German, the spoken past tense (Perfekt) is made of two parts. The helper verb (hast) takes the normal verb position, which is at the front for a yes/no question. The past participle (gesehen) is kicked to the very end of the sentence.

Since this lesson is about verbs that do not get a ge-, why does gesehen start with ge-?

The verb sehen (to see) is a standard base verb. It is not an inseparable verb (like besuchen) and it does not end in -ieren (like studieren). Because it is just a normal verb without those special features, it gets the standard ge- prefix to form the past participle.

Why does gesehen end in -en instead of -t?

Sehen is a strong (irregular) verb. While regular verbs end in -t in the past participle (like gemacht), strong verbs usually keep an -en ending. You just have to memorize these irregular forms as you meet them.

Does das Foto change its form because it is the object receiving the action?

No. Foto is a neuter noun. Neuter nouns use das for the subject (nominative), and they stay exactly the same when they are the direct object (accusative) receiving the action of the verb.