On perl side, use the in-memory open for the image file. C code needs a different import method; should be a trivial modification. Like this perhaps (completely untested):

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);

    ...
}
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