I need to put customized product image in checkout page and order details page rather than original product image. But I didn't find the liquid files in my store theme code.
How can I access these files or any alternative way of doing product image change in checkout and order detail page?
There are some basic options available in the checkout via the General Settings customization ( font-family, colors and backgrounds ).
If you are looking for larger modifications ( html/css/js ) your only option is to buy a Shopify Plus account ( which is around $1000 per month ) and you will have access to the checkout.liquid file where you can modify the css/js and some of the html ( the form fields html structure can't be modified )
As for the single order page, the file is located in customer/order.liquid. You have full access to modify what ever you like in it.