Use the free Irfanview program to resize a similar picture or background to 1280px by 1124px min.
Save As Web jpg to reduce upload size and retain a lot of quality.( An Irfanview plug-in)
Use your Word press dashboard to uplaod the jpg to (default) http://yourblogurl/wp-content/uploads/nameof.jpg/
To install as background go to your Stylesheet [theme editor] and look for body background Leave the color code there as white[#ffffff or equivalent] and add
Don't forget the ; at the end.
Save the Styleshhet and look at a refreshed blog page.You may have to clear your browser cache and/or hit ctrl-F5
If no change ,add quotation marks to the inserted url.
I would have to check my css to make sure.
Meanwhile,look at my blog to see the effect.Change the browser window sizes.It looks best at 980px wide.I tried it in several different browsers at 980px,the accepted blog width norm.
If you need help use my blog contact page.