You can add pictures to your collage in multiple ways.
Go to Pictures tab on the left side of the application.
Press the + button. This will open a file selection window.
Select the pictures you would like to add to your collage.
Go to Pictures tab on the left side of the application.
Drag pictures into the panel inside the Pictures tab.
Open Finder and create a new folder.
Drag pictures that you want to add from Photos app to the new folder in Finder.
Add pictures from folder to TurboCollage.
Method 1:
Go to Pictures tab on the left side of the application.
Open iPhoto and drag pictures from your photo albums in the panel inside the Pictures tab.
Method 2:
Go to Pictures tab on the left side of the application.
Click the + button, this will open a file selection window.
Select photos from the MEDIA category on the left in the window that opens.
You can add individual pictures as well as directories containing pictures.