Can you take a picture of what you are referring to by any chance? I don't think I've ever seen a solid pink background before. I've seen
this before. Is that what you are seeing?
In general, you can set a custom menu background if you are a
supporter.
osu! has seasonal fanart contests, most recent was the
Summer 2018 contest. I'm assuming the images you are referring to were part of that as some of the images from there are used as menu backgrounds in-game.
I
think the summer rotation has ended, but I'm not entirely sure. Check the options menu to make sure the "seasonal background" setting is set to "Always". If they appear then you are good to go. If not, the
Halloween 2018 rotation will probably be coming soon.