How to show thumbnail images on product pages using ubercart module?
Firstly go to [your-domain]/admin/config/media/image-styles and create image style you want to use for your product images, than go to [your-domain]/admin/structure/types and click on 'manage display' on the right (for content type that you are using for products).

You will find your image field whose format is probably 'Image' and you will see a little gear on the right, click on it and for 'Image style' choose image style that you created previously (as mentioned above).

That's it. Now the images will be shown according to image style that you created.

For the list of the product I recommend that you use Views module and again to control your display image size by choosing image style for image field in Views.