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
Similarly the possessive articles ma and sa become mon and son before vowel sounds.
In addition to Stéphane's answer, I would say that it is very similar to the "n" in "an" before a noun in English.
You say "an action" or "an elevator" because it's easier to enunciate than "a action" or "a elevator".