perhaps you could provide a screenshot?

this is done by pressing "Prt Scr/Sys Rq" buttong above Insert and Delete and next to F12

press that button, and then open Microsoft Paint
select Edit> Paste
then File> Save As...

Save the image as a .jpg/.jpeg file (a .bmp will take way to much space) and then go to
and click Browse...
find the file you just made
select it
click upload
then post the link it returns (the one that is just a direct link) here and I can see what your trying to tell us all...

aside from that it sounds like you have the image you want centered in the screen, and the border is bothering you

in that case right click on the desktop
select Properties
go to the Desktop tab
and then in the Position Pull down select Strech or Tile