Without an example we have to guess where the img element in your Passage is in relation to the text you want to display it with. I will assume your Passage looks something like the following:
Some text\
<img id="fjord" src="http://www.gstatic.com/webp/gallery/1.jpg">\
Some more text\
<img class="action" src="http://www.gstatic.com/webp/gallery/2.jpg">\
Even more text
note: any CSS you choose to use should be placed within your story's Story Stylesheet area.
There are a number of different ways you can use CSS selectors to target a specific img element or even a group of such elements. The simplest two being:
1. HTML element ID - like the fjord ID added to the first image in my above example.
#fjord {
border-radius: 1em;
margin: 1em;
}
2. CSS class name - like the action class added to the second image in my above example.
.action {
border-radius: 1em;
margin: 1em;
}
If for some strange reason you don't want to use standard HTML/CSS selectors then you could even use a named hook as the basis of CSS selector. eg. The following Passage content:
|outside-images>[<img src="http://www.gstatic.com/webp/gallery/3.jpg">]
... creates a HTML structure similar to the following:
<tw-hook name="outsideimages"><img src="http://www.gstatic.com/webp/gallery/3.jpg" data-raw=""></tw-hook>
... notice that the dash in the named hook's name as used in the Passage is removed in the name given to the HTML element. The above HTML can be targeted by a CSS selector like the following:
tw-hook[name="outsideimages"] img {
border-radius: 1em;
margin: 1em;
}