Can someone please help w/ my header and tabs?

Can someone, ANYONE PLEASE help me? I am working on a website for a non for profit puppy rescue and I am stuck ?? I know a little about code but for some reason wordpress is extremely hard for me to figure out. The site is https://www.rmpuppyresue.org/site I am trying to change the header to a jpg file I uploaded into WordPress. The url for the image is https://www.rmpuppyrescue.org/site/wp-content/uploads/2011/10/RMPR_BannerLT.jpg. I have read for the past 2 hours different directions on how to change this and for some reason my code doesn’t look like anybody’s posts/directions ?? I have tried placing it in many places but NOTHING is working. I will past my CSS below this message so maybe that will help someone tell me where I need to put the url to the image.

The other thing I really need help with is changing my tabs/pages (Volunteer, Foster, Contact Us, etc) on the top of the page. I need them to be smaller and not wrap below. I need to add more and I don’t know where to look to get it to stop wrapping and I’m not sure if I can make each of the tabs a certain width.

I appreciate ANY help/suggestions. Here is My css:
Thank you!!!!

[CSS moderated as per the Forum Rules. Please just post a link to your site.]