This error pops up for many odd reasons. I'll try to cover all of the reasons that I have discovered.
Too Large - Some phones don't allow uploads over 50k, so you need to shorten your sound sample or decrease the size of the image you are sending.
Invalid Image - Make sure your images are not saved with progressive encoding. Also, you do not want EXIF information saved in the .jpg file.
Bad GCD File - Double-check all of the information there (Content-Size, Content-Type, etc.) and verify that the GCD file only has LF newlines (Unix/Linux style) and not CR/LF (DOS/Windows style) or just CR (Macintosh). (This won't happen if you are using this uploader.)
Midlets - The Manifest file should only have LF newlines. Additionally, the name, vendor, and version must match identically in the .jad file as it is in the Manifest file. Also, case is important – the manifest file must be called META-INF/MANIFEST.mf. (The uploader just generates its own .jad file from the Manifest file, and doesn't care about the case of the file, so it is a bit more flexible.)