It is possible, but the devil is in the details that you haven't included in your description of this problem.
In your table of shipping methods where you have one record for each method, you can add a container field and insert the appropriate image for each shipping method. If multiple methods need to use the same image, you may want to link it to a related table of images where you insert the image into container fields in that related table.
And you can link your Invoices table to your shipping methods table in a relationship if you have not already done so, then you can add the container field directly to your invoices layout and it will show the image associated with the shipping method that you select for that invoice.
Thank you PhilModJunk,
I have currently tried doing this (see picture) with a container field added to shipping_codes but when i try adding the box to our invoice pages it simply says <unrelated table> and isn't showing the image!
Which would suggest that you haven't correctly modified Manage | Database | Relationships to link this table to Invoices.
Thank you PhilModJunk