Thanks for writing to Just Answer.
Should you be upset? No. Should you be cautious? Yes.
Your girlfriend seems to be taking your relationship somewhat casually. Most women would understand that a picture of them in bed with another man would be, at the least, disrespectful to their partner. Why would she be putting herself in situations where getting into bed with two other people is even a possibility? It at least suggests that there is some sexual innuendo going on with her male friend.
If your relationship is somewhat casual, then this may not be an issue. but if this is a woman you're thinking about marrying someday, then it may be time to have a talk about acceptable behavior that respects each other and the relationship. There's nothing funny about posting a picture of herself in bed with others, unless they were all in Halloween costumes.
Make sure the two of you are on the same page about where your relationship is going. If you're serious and she's not, it's better to find this out sooner rather than later.
I wish you the best,