Goodness, you are in a tizzy! My sympathies, I know you must feel very stressed! I know what it's like to feel you can't pin yourself down to one direction, but in this case, you really don't have time to be waffling. I think your supervisor is right; your original idea was very good and you should proceed in that direction. Looking at "how men and women are portrayed in advertisments" sounds awfully broad to me. What if you just buckle down and decide, "I'm going with my first plan"; then, do just what your supervisor said, and come up with research questions. For example, is there a difference in the way different minorities are portrayed in these consumer magazines? Is one ethnicity's portrayal of feminity stronger than another? Can you find specific trends which seem to hold true across the board? For example, are Asian women portrayed as more delicate? Are black woman shown as appearing stronger? Are there stereotypes which need to be addressed? You may find no differences at all, but you won't know unless you look for them.
Have you tried looking online for magazines? I don't know how accessible those types of magazines are on the web without buying an annual subscription, but it might be worth a look.
I know you feel you've backed yourself into a corner, but you still have time to at least do a preliminary draft, if you just go in one direction instead of twenty! :-))
Best of luck!