change existing images

we have around 50 proudcts, and each product has approx 8 images, some few more, some few less. the products are RinGs with Round center diamonds on them. we need to change them, with princess cut (square shape center diamond). these are very high quality images and we need someone who can do this work with the same quality. i am attaching sample images for one product for you. also if you can change the white Gold on these products to yellow Gold. all other background images etc would remain the same without any changes. we need this done very fast. when placing your bid, please provide us with a sample of few images from the sample i am attaching you, to show you can do this. also let us know your best price and time.