ANIMATION SHOP 3 HELP!!!

I HAVE SEVERAL PNG FILES I WANT TO COMPINE THEM INTO A ANIMATEDGIF...BUT THE BACKGROUND IS PURPLE OR WHITE! HOW DO I MAKE IT TRANSPERANT? PLZ HELP

OK I FOUND OUT HOW TO MAKE INVISIBLE..BUT KNOW WHEN I MAKE IT INTO AN ANIMAITON IT SEEMS TO ADD EVERY FRAME BEFORE THE NEXT FRAME SO THE LAST FRAME OF THE ANIMATION ALSO CONTAINS ON THE SAME FRAME ALL THE OTHER'S BEFORE IT..I DOT WANT THAT..HOW DO I FIX THIS?