Changing the "img" tag in the CSS would affect all images, which I doubt is what you want to do.
To create a SugarCube link using HTML so that you can style it, you can do something like this:
<a data-passage="Passage Name" class="link-internal link-image">
<img @src="setup.ImagePath+'Example.png'" style="position: relative; left: calc(50% - (168px / 2));">
</a>
Just change "Passage Name" to the name of the passage that you want it to go to when the user clicks that image. You can set the style property on the image to have the browser display it the way you want it to. This example horizontally centers an image which is 168 pixels wide.
The "@" in front of the "src" property (seen above) also lets you use variables in HTML properties (see "Attribute Directive" in the SugarCube documentation). This way you can have "setup.ImagePath" set to "images/", and use that variable so that the browser can find the correct path to the images. Just add the following code in your JavaScript section, and you'll be able to see images both inside Twine and in your published version using the above technique:
if (window.hasOwnProperty("storyFormat")) {
setup.Path = "C:/prototypesandwireframes/"; // Running inside Twine application
} else {
setup.Path = ""; // Running in a browser
}
setup.ImagePath = setup.Path + "images/";
Note: There was a bugfix for attribute directives in SugarCube v2.23.5, so if you have an older version then you may want to download the latest version (currently v2.27.0) and install that.
Hope that helps! :-)