Yup that's it : I convert the image in PNG, so it can be opened through QPixmaps
The conversion is a little time consuming in high resolutions, but for thumbnails it works really really good

I'm currently using it through QGraphicsView to make a simple image editor (with basic layers), and so far it looks fine.

For MagickWand :
I had some problems at the beginning of my development (2 years 1/2 ago from now) with Magick++, mainly compilation and dynamic linking with MinGW...
I couldn't use static linking in those days, couldn't use Visual Studio too (don't like it ), and dynamic linking was strictly forbidden with MinGW.

So I switched on MagickWand : really simple to use too, maybe too simple, but it seemed loadings were faster with MagickWand...I can't tell anymore, it's pretty similar.

If you can (want) to link statically to Magick++, you can use it, since it's object code, well fitted in Qt

Hope this helps !

Cheers,
Guilugi.