had the same thing hapen to me..

Madeline & Betty,
If downloading png files, convert/save as jpegs & try again.
Don't know about most programs but my program will only work with masks saved in the "jpeg" format. I can leave the area around the mask transparent & add backgrounds or if I wish, which is rare, I can fill with colour. You should also be able to invert the masks in your programs if center is black to put it to the outside.
Again I am not familiar with your programs used but believe jpegs may be your answer. My Photofiltre forum members/myself make tons of masks as do Deviantart members and many other sites.
Let me know how you make out - am curious. If things don't work out, don't hesitate to pm me & I will try to help in any way I can.

I don't think this is going to be if much use to you but when I make my own masks I always save them as gif and although they look black, as soon as I add my photo to them the photo always shows through. Masks always appear black but if used correctly the photo shows through the black. If your programme has a layers panel make sure your photo layer is active and just above the mask in the panel - here I press Control+G and they merge together. Can't help with other programmes.

Thank you all - I will give all advice a go in Paint.net- wish me luck LOL xxxxxxxx