A while ago I tried to upload an animated GIF to this blog, and found that the upload would fail with an excessively vague error. This seemed to be related to the upload being an animated GIF. I searched for other WP users having troubles with animated GIFs and found that the only complaint seemed to be that the reduced-size images automatically generated by WP were just the first frame of the animation. There is a WP plugin which claims to properly resize animated GIFs but I have yet to install it since my problem seemed distinct from what other users encountered.
For now, what I did was take the still image of the first frame and upload that to WP. Then I located the file under the wp-content folder on the web server (since I did this in December 2013, it was in wp-content/uploads/2013/12) and noted the resized versions in the same location. I then resized the gif myself to the same sizes WP generated, and manually copied the animated GIF files to the corresponding files on the web server.
However, I just tried to upload the animated GIF again to see what that vague error message was, and now I find that the file does upload properly, and, as observed by others, the reduced-size images are not animated.
So maybe it is time to try that plugin…
One other interesting observation is that the original upload file and the resized ones do not have the same group under the wp-content/uploads structure (this is using Apache on OpenBSD 5.3).