Can you take a screenshot of what your Gallery map looks like? The snake scene should automatically appear when the map is unlocked in the upper right corner. Is there just a blank space between the slime queen sprite and a slimed up Mira?
Beat the first boss, trigger the cutscene above them, then use the Key Item, Ankhrono Replica, to access Somewhere Between Time and Space where the gallery is the room to the left.
There is also a walkthrough if you have more questions.