To upload your own images, click the first square to the left (labeled "Upload image") and select the image you'd like to upload. Instead of using the file browser to select the image upload, you can also drag a file from your computer onto that first square.
After this, a preview of the image will appear in the leftmost square. At this point, your image has not yet been uploaded. To continue and upload your image, click the upload icon as shown in the screenshot below:
You can also choose to crop the image before uploading it; this can be done by clicking the edit icon:
Clicking the edit icon will bring up a modal popup with drag handles around the image allowing you to crop the selected image.