Customer image upload function for personalised gifts

I run a website called Canvasmart.co.uk, the platform is a ecommerce template from prestashop,
My website allows customers to upload their own photograph with s product which is then printed by ourselves.
The actual function to do this is already in place but it is not working coorectly as when the image is uploaded nothing is happening with the file.
It should thumbnail the image to show the customer their image in the cart with the product that they have ordered.
As all of this function is already in place I have been told that the problem lies within the script of my website which is stopping the upload function from working properly.
I need someone who can look into this problem for me and advise or fix the problem.
More details will be given upon request