during work on 6480688 I've found that ImageWriterSpi.canEncodeImage() may be too optimistic
about writer's ability to encode image. I.e. as far as I understand gif writer may not be able
to write image with tranlucency, but even the following check

imageWriter.getOriginatingProvider().canEncodeImage(new ImageTypeSpecifier(renderedImage))

where:
 imageWriter - a writer we are going to use
 renderedImager - RenderedImage we are going to encode

usually returns true, but results of write/read of this image may not be very successful :(

The attached test demonstrates this problem for gif writer.
Left canvas contains original image, right one image after write/read.