All images that are uploaded to the HTML5 editor as assets can be made localized, meaning that you can display different image based on what language the banner is in.
Hover your mouse cursor over the image asset you want to make localized and click on the wrench that appears after a second or right-click on it and then click on Edit.
In the image settings, select which language localization you want to set a new image for and then click on "Upload" to open the file system and browse to the image you want to use. The image you upload will be displayed in the selected language instead (in this example the BannerFlow butterfly). Save the image settings, save the banner and check it out in the showcase.
Don't forget to hit save when you're done.