Taking a picture of your in-game character posing with Pearl or Marina on stage during Splatfest events is easy. All you need to do is go to the giant Amiibo box in Inkopolis Square and scan your Peal or Marina Amiibo in like you normally do. The Amiibo will ask if you'd like to take a picture with her. When you confirm that you would like to take a picture, you will appear on stage with your selected character. You can select from different poses for both your player character and Marina/Pearl, just like you normally can when you take a picture with them.
The only real difference between taking a regular picture and taking an on-stage photo during Splatfest is that the background is changed to the Splatfest concert during Splatfest.