When you press the "+" block, you can add images, which will appear as preview. In the screenshot below, the last photo (3) is a preview.
How can I tell the user that it's not an uploaded photo but a to-upload photo?
There are many options for you to try and see which one(s) fits best with your design style and implementation requirements.
You should probably also consider if there are other states that the object/image can be (or define them) so that you can also represent them consistently and clearly.