This trick works with any OS: Linux, mac or windows, as long as you have thunderbrid.
To change the home page, which usually represents the Thunderbird logo, just go
Tools> OPTIONS >General tab and the section: thunderbird start page, in the Address field mention the website address that you want to appear in the Start page, without forgetting the http://.
If the first solution doesn't work:
Open Open Office (or equivalent), create a new HTML document, then write or put an image.
Save your creation. Open the page in your browser: copy the address. Paste it into the address field. By cons, it is strongly recommended to move the file once back in thunderbird, because if you read, you will see the address of your page does not [http://] but it is rather like: