When you send a proposal from Better Proposals, it comes in a beautifully designed email with your logo at the top. This logo should be suitable for a white background.
Change the logo on your client emails
To change it, click Settings > Proposal Branding > Email appearance
From there, click 'Browse for logo' and replace it with the logo you want to show in your client emails.
It's best to use a transparent PNG with a minimum width of 400px.