I have recently had an experience under a shared project with a graphic designer who I believe has experience mostly in traditional print and not web/apps. Although the designs were of quality as far as layout etc, the image quality was the biggest issue. Most were delivered in the incorrect sizes, quality, aspect ratio and not a single of them had been tinified (tinyfy)for web.
Generally I have a rule where I only use my own designers and I will be envoking this rule again with no exception in future.
It makes the whole experience frustrating for all as they will take no responsibility for delivery of the appropriate assets.