To insert an image you should use the standard HTML line <img src=”image.png”>. You can find some examples using HTML in the Arduino IDE under File > Examples > Ethernet. I haven’t find any sketch using CSS, but you can check this link where you will find some examples of CSS for the Arduino UNO Ethernet Shield.
As far as I understand, you just have to call the HTML file to open it (including its style sheet and/or images). Or you can start printing HTML instructions directly from the sketch, the example WebServer shows something similar.