AppStyles.java

public class AppStyles {

    private static final Map<String, String> imageUrlToCssClassMap = new HashMap<>();
    private static int counter = 0;

    public static String getCssClassNameFor(String imageUrl) {
        if (imageUrlToCssClassMap.containsKey(imageUrl)) {
            return imageUrlToCssClassMap.get(imageUrl);
        }

        String cssClassName = ".makeYouUniqPrefix-" + counter;
        counter++;

        StyleInjector.inject(cssClassName + "{background-image: url(" + imageUrl + ")}");
        imageUrlToCssClassMap.put(imageUrl, cssClassName);

        return cssClassName;
    }
}