45131 OSCommerce Image Uploading

Basically I am implementing a store based on OSCommerce which produces posters and prints - the store initially will just do stock prints so the user picks an image they want from the catalogue, then a size and paper type and its printed and sent. Easy stuff! :) However, the future plan is to allow users to upload their own digital images for printing - as before the user picks the size and paper type but this time its under an item called yourprint or something similar. Once they complete checkout (i.e. payment is made) I want to be able to then allow them to upload the image to my server so I can print it for them. Each image would need a unique reference and be able to be assigned to an order - I would also like to be able to restrict the size of the image (physical weight and dimensions) as well as the file types which can be uploaded. The important bit is I want to make sure the upload only happens once checkout is complete to stop people abusing the system and trying to bring the server down by uploading lots of big files at the same time or uploading harmful files etc - I dont want them to try and upload the file as part of the checkout process. I know I could ask them to email me the file but this seems slickers so is this something you could help with? I want to make sure the upload process is as safe and fast as possible. Please reply with timescale, price and examples of previous OSCommerece customisation / modifications.