Afaik, QBO doesn't offer such feature. You may utilize a free invoicing app with the e-sign feature to do so and utilize a connector to mirror the invoice data back to your QBO account as a workaround.
Thank you for posting in the Community, prayank-vidyarth.
Let me add some information about attaching signatures for invoices in QuickBooks Online.
I agree with @Fiat, you may use third-party application to attach signatures for your invoices. Since, the feature is currently unavailable in QuickBooks Online.
Also, I'd recommend sending us feedback or any product suggestion. This way, we can improve your experiences while using QuickBooks.
Our product development team will receive and review your suggestions and may include it on the next updates.
Please let us know if you still need further assistance. I'll be around if you need help. Have a great day!