The image picker accessed via the + button only allows you to select one photo. This is a limitation of the built-in iOS image picker which does not currently provide support for multiple selections.
However you can add multiple images at once from your Photos app by via copy/paste or, if you are using an iPad, via drag & drop.
Method 1: Copy/Paste
- Open your Photos app (or your other app such as Google Photos)
- Tap on Select
- Select multiple photos
- Tap on the Export button
- Tap on Copy
- Return to Print to Size
- Tap on a blank part of the page
- Tap on Paste
Method 2: Drag and Drop on iPad
See this page to learn how to use this approach on iPad.
Note: Because iOS only supports drag & drop across apps on iPad, this method does not work on iPhone and iPod Touch.