You can easily change the logo image that displays in emails, on generated invoices, and throughout the Client Area.
Change the Logo via the Setup Wizard
To do this:
1. Navigate to Configuration > Setup Wizard (Help > Setup Wizard prior to WHMCS 8.0).
2. Click Next.
3. Click Choose File and locate the .png or .jpg image file you wish to use as a logo.
4. Click Next.
5. If you haven't already done so, complete the rest of the displayed steps. Keep clicking Next until you reach the end of the wizard.
6. Click Finish.
Your logo will now display on emails, invoices, and in the Client Area.
You can also change the logo by updating Logo URL in the General tab at Configuration > System Settings > General Settings (or, prior to WHMCS 8.0, Setup > General Settings).