Host an image
Your own branding/logo can be applied when you purchase signatures.
Simply find the image you like, right-click it, and choose "Copy image address"
However if you don't have a website, or want to use a different image than what's available on your website, you can take advantage of Google Drive Drawings:
Create a DRAWING
Open your Google Drive
Click + New
Hover on More
Click Google Drawings
Create your drawing
Use the functions like shapes, text, and upload to make your image.
Tip: If using Upload to bring in an existing image, ensure the file you're using is PNG format.
JPEG isn't appropriate as they just appear black in the next step
Publish to the web
Click File
Click Share
Click Publish to the web
A panel appears where you can set the desired size, and Publish.
Copy the link you're provided.
Now you can use that link when purchasing signatures!