Isn't the pronoun supposed to match the noun genre? If "image" is feminine shouldn't we use "ta"?
Ta is generally used for feminine nouns and ton for masculine nouns… but before a vowel sound, ta is never used. In French hiatuses are commonly avoided by resorting to elisions or other grammatical artifices.
Thus, “ton image”, pronounced
/tɔ̃.n‿i.maʒə/, just like “ton idée”, “ton ubiquité”, “ton histoire”…
Similarly the possessive articles ma and sa become mon and son before vowel sounds.
“Ton image” is correct and not “ta image” because in French, the possessive adjective has to agree in gender and number with the noun it modifies, not with the person who owns it. However, when the noun is feminine and begins with a vowel or a silent h, the masculine possessive adjective is used to avoid a hiatus. A hiatus is an awkward sound that occurs when two vowels are pronounced one after another. For example:
Ton image (your picture) is correct because “image” is a feminine noun that begins with a vowel. Using “ta image” would create a hiatus between the two a’s. Ta montre (your watch) is correct because “montre” is a feminine noun that begins with a consonant. Using “ton montre” would not agree with the gender of the noun. Mon amie (my female friend) is correct because “amie” is a feminine noun that begins with a vowel. Using “ma amie” would create a hiatus between the two a’s. Ma voiture (my car) is correct because “voiture” is a feminine noun that begins with a consonant. Using “mon voiture” would not agree with the gender of the noun.