How do i insert an different image through formula field from documents to an product object

How do i insert an different image through formula field from documents to an product object

If i go with this 

IMAGE("/servlet/servlet.FileDownload?file=015i00000001hVq", "Laptop" , 100, 100)

The same image come up for all the products.

The another way which i have tried is by creating an field like imageid on the product object like and created the formula

IMAGE("/servlet/servlet.FileDownload?file=" & Product_ImageId__c , 
Name,50,50 )

But the above problem is that i need to manually code the id and paste it in the field of imageid soo that the above formula works out.

Is there any other way to fetch the document id by matching the product name and the document name which displays the image.

Thanks in advance who guides me out.

Thanks,

Atif