welcome to the forum and I think this question has been asked before. I presume its this :) How to change the blue background image behind the SharePoint 2007 Page
also this is a nice read to change the masterpage from sharepoint designer: http://office.microsoft.com/en-us/sharepoint-designer-help/modify-the-default-master-page-HA010100906.aspx
BTW I think its a css issue not a masterpage issue... below outlines what you want, above is to help you understand the consepts more ;) . ALways Always Always save the origional of anything your changing like masterpages, css, web.config..... If you make a mistake you can revert back :) .
also this will point you into the right direction into removing all the images that you dont want, including the blue background... check it out here:
How to create a Sharepoint 2007 clear, simple and consistent theme and get rid of the default gradients
Hope it helps :)