I can't get my image to center

I am trying to get an image to center in my tribute page. It won’t work and am a bit confused on what the code needs to look like. Do I put align=“middle” for this or is it something else? Thanks

for images in css file write
display: block;
margin: auto;

for text and buttons it will be
text-align: center;

Thanks for the help.

Is there a way to do it in bootstrap? I am a little confused on when to use bootstrap and when to use CSS

Bootstrap has center-block class, iirc, which can be used for this.