Hi David,
I am afraid that my knowledge of Joomla! is rather lacking compared to my colleagues, but I've been playing with Startup and I may be able to point you in the right direction, should my colleagues have not already provided assistance:
1. You may remove the 'Sign In' button in the template options; from the Joomla! Admin Panel navigate to Template Manager>gk_startup>Features and disable the 'Register Link' option.
2. A quick way to change the background image is to replace the image files directly with your preferred images; note that there are 3 different images at varying sizes to meet the needs of a responsive layout, so please do try to ensure that you maintain this ratio to ensure that the image scales correctly when viewed on tablet/mobile screens.
The files that you should replace with your own images are located in the templates/gk_startup/images folder of your Joomla! installation, and the file names are as follows:
header_bg.jpg
header_bg_mobile.jpg
header_bg_tablet.jpg
Obviously the suffixes refer to the screen size that the particular image will appear on i.e. header_bg_mobile.jpg will appear when the site is viewed on a mobile screen. I am sure that my colleagues will be able to provide a more effective, less 'dirty' fix, but I hope that this will suffice in the meantime.