Imagemixer does not have a good name for support or ease of use - see the number of threads on this and other forums asking for help with it.
For my recommendations for reasonably priced yet effective video editing software have a look at these two:
Adobe Premiere Elements 7
A very efficient and powerful editing suite that allows multiple audio, multiple video tracks (and therefore layering with differing opacity), Picture in Picture, green-screening, many transitions and effects, ability to burn straight to DVD from within the software, post to Youtube from within the software and much more.
Vegas Movie Studio Platinum Edition 9
again multiple tracks for audio and video (only 4 of each - compared to Adobe's 99), Picture in Picture, green-screening, many transitions and effects
Both accept input from a range of file formats and output a similar range, both are relatively easy to use but offer powerful editing capabilities.
I prefer Adobe - mainly because I started on Adobe and have used Photoshop for years so am familiar with Adobe layouts and terms.
Download a trial version and try them out - best way to see which suits your way of working.