open(PNG, '>', \my $img_data);
...; print PNG ...
process_image($img_data);

__C__

SV* process_image(SV *image_data) {
{
    STRLEN dlen; SV *data = SvPV(image_data, dlen);

    gdImagePtr im = gdImageCreateFromPngPtr(dlen, data);

    ...
}