Check if your image data contains some header information at the beginning:
imageCode = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAMgAAABkC...
This will cause the above error.
Just remove everything in front of and including the first comma, and you good to go.
imageCode = "iVBORw0KGgoAAAANSUhEUgAAAMgAAABkC...