You can upload a logo or other image to a service like www.postimg.org then you can follow the instructions below to add and update the logo.
Adding a logo:
1) If using postimg, first upload the logo at posting.org
2) Copy the direct link (it should look something like http://postimg.org/fuhgboueh/yourlogo.jpg)
3) In any section of the letter or email creator on Tonic (usually the header) on the menu click "Insert > Insert Image"
4) Paste the direct link you got from postimg into the source field add a description if you like and leave dimensions blank
5) hit ok and the logo should appear.
Updating:
1) First select the logo as you would a block of text
2) Click "Insert > Insert Image" the dialog box should appear populated with the current image
3) Adjust the dimensions, description as you see fit, use constrain proportions to ensure the logo isn't distorted. You can also replace the logo by adding a new link to the "source" field.
Other sites you could use are imgbb.com and flicker.com
