You can create it with the Document class of GWT and cast the Element to your new Element class.
myImageElement element =(myImageElement) Document.get().createImageElement();
But, in GWT all *Element
classes are a method as for doing the cast :
public static myImageElement as(Element elem) {
assert elem.getTagName().equalsIgnoreCase("img");
return (myImageElement) elem;
}