Thanks Ray for your help - I have found a solution to my problem:
I was using the code below which converted all image types to JPG which distorted the colors of PNG files:
<?php
WA_DFP_SetupUploadStatusStruct("WA_UploadResult1");
if($_SERVER["REQUEST_METHOD"] == "POST"){
WA_DFP_UploadFiles("WA_UploadResult1", "address_image", "2", "[NewFileName]_[Increment]", "JPG:80", $WA_UploadResult1_Params);
}?>
I have changed the code to this one which uploads the images in their original file type:
<?php
WA_DFP_SetupUploadStatusStruct("WA_UploadResult1");
if($_SERVER["REQUEST_METHOD"] == "POST"){
WA_DFP_UploadFiles("WA_UploadResult1", "address_image", "2", "[NewFileName]_[Increment]", "true", $WA_UploadResult1_Params);
}?>