When I execute it in ezpublish root, I have the following output :
convert : no decode delegate for this image format 'var/plain/storage/images/image_1/183-4-fre-FR/image_1.png' convert : missing an image filename 'PNG:'var/plain/storage/images/image_1/183-4-fre-FR/image_1_reference.png'
and I've got file 'var/plain/storage/images/image_1/183-4-fre-FR/image_1.png'