I've been having same issue since Nov 5th. It's about 50% of the time for me. I send the delay option in the url and set the number of images to 5 for the animated gif. The first 4 images are fine but have no delay between them and the last image has black background. When it works like it is supposed to there is a half second delay between images in the animated gif and the background is normal.
I received an answer from Wunderground support. They try to reproduce and fix the issue. It seems very intermittent, which is also what I experience. Hope they find the issue soon. If anyone experiences the same issues, please post it in here (with the API URL used). This may help them find the issue faster.