Can I change the way a single image looks on my site without affecting all the others?

Yes you can.

The ‘Additional CSS’ functionality in the customizer is great if you want to change how an element, like an image, looks on the whole site. But what if you only want to change the look of a single occurrence of that element? 

To do this you need to know a little bit of html and a little bit of css.

For example, your images may usually have square corners, but what if you want one of them to have rounded corners?

Go to the post or page you want to edit, then click on the Text tab of the editing area.

Find the image you want to edit – it’ll be a string starting ‘<img …’ – then after ‘img ‘ add ‘style=”border-radius: 5px” ‘. 

That will round the corners of just that single image, leaving all of the others on site with their square borders.