Go to: Setup → Custom Settings → Invoice settings → Manage → New/Edit
Image are represented as Static Resources (Setup → Static Resources → New)
Image can be uploaded directly as Static Resource. In this case you just need to specify the name of static resource in Invoice Settings Custom Setting
Image can be uploaded as a part of Zipped archive. In this case Name of Static Resource and path to image must be specified in Invoice Settings Custom Setting, i.e. My_Static_Resource/my_folder/my_image.img
Configure text in the page
Enable Translation. Go to: Setup → Translation Language Settings → Enable.
Go to: Setup → Custom Labels → Open any Custom Label related to the Invoice Category -> New Local Translations / Overrides → Enter any value for label.