You will add images as you're creating a new item or when you edit an item. It's located at the bottom of the Create/Edit item page.
Step 1) Navigate to the "All Items" page.
Step 2) Scroll down to the bottom of the page.
Step 3) Select "Add/Edit images" you will be directed to a screen to add the images. You can drag and drop images into the box or click Browse to add from you files. Hit Finish & Back to save the item.