I use "XN convert"
When it opens click the "ouput" tab up top and change format on the right to "png"
then click back to the "input" tab and select the files or folder you want down at the bottom left, then click "convert" down at the bottom right.
MASSIVE WARNING!!!!! Unless you have a shit hot computer DO NOT select to convert the entire unpacked folder at once because it will take a LONG time. Individual characters wont take long depending on who it is. also the backgrounds should be fine already.