Why won't my image border go away?

border property is not supported in HTML5.
In your stylesheet either remove border property or make it none

body.page .hentry img, body.single .hentry img {
    padding: 7px;
    border: solid 1px #dddddd; /* REMOVE THIS */
}

I can’t find that code. There are 19 css files for this theme ??

Line 229

https://www.youracademicassistant.com/wp-content/uploads/montezuma/style.css

It’s not a good idea to edit the theme files directly, as you’ll lose those changes if the theme is ever updated in the future, to fix bugs or security issues or to add new features. Instead, you should use this code in a custom CSS plugin or your theme’s built-in custom CSS option, if one exists:

body.page .hentry img, body.single .hentry img {
	border: 0;
}

It worked!!! Thank you, Stephen!!