SharePoint background image pixelated

[randle]

Hi,

 

I've been designing a site within Office 365 SharePoint and so far all's gone well however, I've uploaded a picture to use as the background (jpg 1600 x 1200 180 dpi) which SharePoint zooms in so that the whole page is filled. The problem is that the uploaded picture is extremely pixelated compared to the source file at the same zoom level. Not sure how I get round this?

 

Maybe there's a restriction on file size but SharePoint isn't complaining about this, so as far as I can tell this isn't the problem but looks like the quality is getting automatically reduced at some point during the upload!?