Yes, "wenn" only means "if". English "when" (unless you can replace it with "if" in English) corresponds to "als". There is also "wann" (interrogative and relative pronoun).
wenn = used for a general condition that could apply more than once. Mostly used with present tense.
als = used for a specific condition that was true at a specific moment in time. Mostly used with one of the past tenses.
"Wenn du eine schöne Frau siehst, ..." implies a general condition (everytime if/when you see her), so "wenn" is correct. You can translate it both as "when" and "if" into English.