It would be hard to manipulate a traditional drop down box via CSS injection to add image. This widget lets you add and select images as drop down box. The only twist is that it is an Android style and design which is fun!
Simply search and drag the Android text & image selector widget on your forms! The wizard can be confusing at first but all you need is a title without quotes and a direct link to your image which is separated from the title using double colons. The format is Title::imageURL. If you need two or more you can separate them into commas. Title::imageURL, Title::imageURL
JotForm's select box widgets give you stylish options for creating select boxes. Whether you'd like respondants to be able to drag and drop an orderable list or select a terms and conditions box, you can find the right widget for your needs.