Post header image

I would like to be able to establish featured images in each of my articles so that, once listed in “Related Articles” or “Recent Articles” they would appear together with their featured image, thus making them more attractive to the visitor.
However, I do not want them to appear in the post. That is, I would like them to be but hidden. I found a css code for it, but when I hide the image, it leaves a gray background that looks really ugly.
Could someone tell me how to hide the featured image in the post and also not add any background or that it be transparent and the title of the post in black?
Thanks for reading me…