Elle here from Shopify.
You can accomplish this by coding the image into your theme, within the /sections/product-template.liquid.
First, head to Settings > Files and upload your image. Grab the URL that is generated and then paste the following within the template depending on where you'd like the image to appear:
<img src="PASTE THE URL FOR THE UPLOADED FILE HERE">
If you need help with the customization and are using a Shopify free theme, let me know and I can arrange for our theme support team to make this customization for you! When using our themes, you receive 60 minutes of design time on your account.
Hope this is helpful! :)
If you still need to add the real image to all your products, and if you are open to an app suggestion, then you can do it with the Excelify.io app (https://excelify.io).
Import the columns:
For the Handle - fill the Product handles
For Image Src - put the image URL which you need to put.
For Image Position - put the value 0. That will make this image go in front of all existing images in that product.