I was an RA for two years. All the following advice is based on the rules at Monmouth College in Monmouth, IL. YMMV.
1. She can be offended all she wants, but that means little unless you do other stuff she's been letting slide. Things she really could bust you for.
2. If she did want to get you thrown out of the dorms, 75 days is probably not long enough. She's got to do the writeup, convince somebody that you should be kicked out over one offensive poster, then you've probably got 30 days to respond. Even if that goes against you, you can probably then demand a hearing which will probably take another 30 days. You can string it out that way forever. I had people who had bombarded people three floors down with beer bottles and urinated on crowds below their balconies who were safe from eviction because the Dean didn't even want to bother trying.
3. There's a good chance she thinks just saying she's offended will cause you to back down whether you really have to or not. I'd leave it up unless you know she can get you.
4. It's amazing what passes for offensive, isn't it? Suggest pointing a gun at a RAPIST, and you're offensive. Put up pictures of the Jetsons and the Flintstones committing unnatural and wholly disturbing acts in all possible combinations, and you're just a rambunctious college kid.
I was an RA, but I had my own run-ins with the "establishment." My Dean thought my job was to enforce whatever he said, whether it was in the rules or not. HE didn't want to set the quiet hours earlier because it would upset people, but when Himself was working late one Friday night he called me and told me to tell everyone to shut down. This was 10:00 on Friday night. The quiet hours began at 2:00 in the morning (yes, that was idiotic, but it was his idea.)
I went to everyone one by one and told them that it was my duty to tell them that Dean Ambrose wanted them to be quiet and go inside. When they objected, I told them "Hey, I didn't say I cared what you do. Doesn't make a bit of difference to me until 2:00. He told me to tell you, not to make you do it."
Man, it was loud. I hated the noise in that place with a passion, but I enjoyed it that night.