Show an image without any border using Preview or some other built-in macOS app
What you need to do in Preview is to set the fill color of your image to white.  You may have a border but it will be invisible. If your page color is black, then set the background fill as black too.  You might need to set transparency to 100%.  A little trial and error should determine this part.

It is possible to set the Border color and Fill Color of your image in Preview. Open your image in Preview and go to the View menu item and under this menu, you'll see an option to Show Markup Toolbar. Make sure this is turned on. This will display the 2nd toolbar with the options to set the fill color and to set the border color. See the image below.



If you click Show Colors on the menu for setting the fill or border color you will get the Color Wheel to select the color. There is also a setting on the Color wheel for Opacity.