This will depend on how your theme is structuring the templates for single.php (individual post view) but I’ll show you what to look for and how to apply via CSS.
So from a theme I’m working on, this is where the category gets included in the post:
<article id="post-3417" class="post-3417 post type-post status-publish format-standard hentry category-plus">.
The last class in the <article> element is category-plus. So I want to target that.
My CSS would then look like this:
article.category-plus > header.entry-header > .entry-title:before {
content: url('wp-content/uploads/2020/05/C-Plus.png');
display: block;
float: right;
margin-right: 10px;
width: 1em;
height: 1em;
}
You’ll have to check the source code of the one of the pages you want to add this to (because your theme is different than the one I’m currently working on, so it’ll probably have a different set-up), locate where in the entry it specifies the category and then use that information to properly structure your CSS selector.
You can adjust sizing, positioning, etc, as needed – but this is far easier and standard than writing to your functions.php to include a single CSS style rule.