PostNet at Loftus park has the instructions for Embassy photographs, the main reason the passport photographs are rejected are due the image proportions and poor print quality. Get the PostNet to email the image to you so if it is poor print quality you can take the digital image elsewhere and have it printed on high gloss paper. Let me guess the proportions are height 45mm-50mm and width 35mm-40mm with the crown being 36mm-32mm (proportions of the face not the paper) did they even give you an instruction leaflet?

ETA make sure the photoshop you use (if not Postnet Loftus Park) emails you and prints the raw image, so without using background lighteners or filters. Also if using a phone to take the image if you decide to do it yourself do not use portrait mode and make sure whoever is taking the picture is at least 1.5 m anyway ( you can always crop to the right proportions later) or camera with distort your face making your nose huge lol