My memory's a little fuzzy on this, but I feel as if the format doesn't matter as much as you'd think. Seriously, try uploading the .png (or any other file types for that matter) - if NG doesn't complain, you have nothing to worry about, and the worst that could happen is you waste time exporting files.
Otherwise, see if you have any programs other than flash that can convert the .png back into a .gif, adn upload that. If that doesn't work, I dunno what to do. sry. T_T