what do you do if part of the image you want visible IS black?
If you have an image editor like Photoshop, Paint shop pro, or the GIMp program, which can deal with transparency and selection masks. the you would create your Colour image,. and a separate Alpha mask.
you would create a grey-scale image for the Alpha channel which would have all the parts you wanted to be visible in White, and all the parts you wanted to be invisible in Black.
So, for example,. If you had Black text in the colour image, but wanted it Visible when using the alpha channel,. you would make anothe image where the text was white and the background was black.
A simpler method is to use a PNG format (which supports alpha channel transparency) instead of JPG which doesn’t.
Quick example :
colour image with black text and a white Lightning bolt.
Alpha channel mask with the text and lightning bolt masked in white on a black background.
the resulting render on a Plane