First, Check what kind of dimensions and resolution your cellphone likes.
Secondly, adapt the export settings in Powerpoint accordingly.
Thirdly, get yourself a decent graphics editor (there are loads around as freeware even) as Powerpoint is definitely no graphics tool.
How am I supposed to know the abilities of your cellphone if you don't even mention the name?
Anyway: as you have files that show correctly, you could extract the cover from one of these (there is an action for this in mp3tag or do it with the extended tags dialogue) and check the dimensions and resolution with a tool.
Once you have found these properties, create the other graphics with the same attributes.
As for Powerpoint: I would experiment with the page settings.
But as I think that Powerpoint may have its virtues for presentations it is not intended as graphics editor and so if you insist on your workflow I am afraid that I am of little help in this respect. So my only advice is that before you invest any more effort in Powerpoint, try to find a decent tool (as apparently you have done already).