Using the Built-in Letters, you can have your own stationery in the letter.
Simply create a graphic that has your stationery and designate it to be used for you letters.
Creating the graphic needs to be done outside of CMS and CMS does not support this part of the process. Consult your graphic designer for assistance in this.
Once you have graphic file, click Program Setup > Options > Stationery and choose that graphic.
If you're using CMS on a network, the location should be on the shared server.
This is not just for the logo, but can be the entire letterhead and side bar with a background watermark. Use your imagination.
It can work in conjunction with the built-in letterhead or replacing it, depending on the other settings on the Stationery tab (options screen)