Don't feel bad. It wasn't a good taste joke. And what seemed to fly over a ton of people's heads is she had a flak jacket on. Essentially a physical gag about being protected from creeps.
I know everyone focuses on the photo and Learn Tweeden, but there were eight other women who made accusations. This is an old article, but it lists the eight accusations before he left, and there was a ninth after he left, detailed in the second link.
It's not just the one photo. It's that half the women had the same story of Franken touching them inappropriately during photos at public events.
Oof, men being creeps in photos is super common in my experience. I do historical costuming, and men think it's hilarious to pretend to squeeze your boobs or butt (or bustle) when they ask for photo. Obviously the historical clothing adds another layer to the situation, but I don't dress that way to be a punchline to a joke. Most of my costuming friends have similar stories of men oozing into the picture after their wives ask for a picture. My friends who do cosplay have had men do weird shit in photos, too. And they never introduce themselves first, so they can just disappear into the crowd after.
Admittedly that's different than the situation with Franken where other people asking for his picture, but men are comfortable invading women's personal space when it comes to photography. Lots of my female friends who don't do costuming have stories about the creepy guy who gets too close for photos at family or work events. Just because Franken is famous doesn't mean he can't be a creepy dude who takes advantage of the situation.