Border everytime I add an image to a post

Not really as you are using Elementor.

Are you always using size-full images?

If so you can try this:

.single-post .site-content img.size-full {
    outline: 1px solid #000;
}

That took away every border from the images :(

Use this CSS to remove the outline from the Featured image:

.single-post .elementor-widget-theme-post-featured-image img {
    outline: none;
}

Thank you so much!!!!

You're welcome

You're welcome