Multiple selection in a gallery picker

In one of my mobile apps there is an option to edit multiple photos at once.
At the initial screen, the users are presented with a picker view with all of their photos from the gallery. From testing I have made, I found out that people don't actually understand that the picker is a "multiple selection" picker; most of them think it's "single selection".

To try and solve this, I have added a mechanism to preview the users with the already selected photos, but still there is a UX gap that I can't figure out. Here is a illustration of the correct behavior:

Preview of the correct Behaviour

How can I make the multiple selection more obvious for the users?