Sometimes my software creates jpeg images with REALLY big file size, although they are 640x480. So what I do is : I open this big heavy jpeg image with simple Windows (MS) Paint and just Save it (CTRL+S). And close. Looks like nothing happened - the quality looks the same and everything, but the file size can be about 10 times smaller!
Just a little trick I use to make sure I don't upload too heavy files :)