You need an ImageReader. if size is 1 its not animated, everything above is animated. Check this out:
public Snippet() {
File f = new File("test.gif");
ImageReader is = ImageIO.getImageReadersBySuffix("GIF").next();
ImageInputStream iis;
try {
iis = ImageIO.createImageInputStream(f);
is.setInput(iis);
int images= is.getNumImages(true);
} catch (IOException e) {
e.printStackTrace();
}
}