Sexual Harassment?

I'm a dept supervisor at a national retail store. Last week I was eating lunch in the break room, reading the sports page, when three(women) co-workers came in and sat next to me. I then put the paper on the floor to make room. 2 of the women who sat at my table are above me in ranking, and the other was a fellow dept supervisor. After finishing lunch, I put the sports page back on the table and went back to work. About a minute later I was called back to the break room. There was a Viagra ad and a penile enlargement ad on the back of the sports page, so they proceeded to say, "I see what you were looking at." I denied it, and just blew it off. Later in the day several people would pass by me and laugh(One person was an assistant store manager)...I asked them what was so funny,and they simply said, "you'll see." Finally, as I was going home for the day, I checked my mail box...and there was a piece of paper taped to the outside of my box with my name on it stating that I left this in the breakroom..and underneath the writing was the penile enlargement ad that they cut out and taped to the paper. I immediatley went to the store manager(woman), and I said, "can you believe they did this to me?" And all she said was, "I know." A co-worker said that the store manager saw it, laughed at it, and DID NOT take it down! The girl who actually did it, didn't get in trouble, and the person who came up with the idea was the female security manager. I still have the paper with the writing and ad taped to it...and half of the store as witnesses. The people apoligized, but it didn't seem like a big deal to them. I can't sleep at night, and I don't know if I can still work there after all of this. I can't believe that 3 assistant store managers, plus the actual store manager saw this taped to my box, and didn't take it down! Does anyone think I have a case, and can the employer be liable for a potential lawsuit? Any info will be appreciated.