Yes, you can. Pickit users get unlimited access to free-to-use images from Pickit Market at no extra cost.
You add images to your collections by going to Pickit Market when you're inside the Content Administration view. You find Pickit Market by going to "Find Images" in the menu to the left and then by clicking on "Pickit Market".
Once you've found an image you'd like to add into a collection, click on the image and then click on "add to collection" to the right.