There are a few ways to add images to your signature:
1- Profile picture / Company logo:- In the Personal Details tab, there is an option called Upload your Picture, you can upload yours.
2- Company Logo:- In the Company Details tab, there is an option called Upload Company Logo, you can upload the company logo.
3-Banner:- To use a banner for your signature, Go to the 'Apps' tab and select the option given called "Upload your Banner". After selecting it, you can upload the banner and it will be shown in the banner section of your Signature.
(It is recommended to use atleast an 800px wide image)