In order to add your own picture to a template, you need to upload it through the Uploads section of the Graphics Editor.
Just click on the green Uploads button and choose the file from your computer. You can upload .png, .jpg and .jpeg images. The file will appear on your template immediately after uploading. Then you can simply drag and drop uploaded images to the Editor.
All the uploaded files will remain in your Upload section forever. And when you want to delete a certain image, just click on the orange cross icon.