you could use the in built editor to make the logon images the resolution you want. go into logon studio, then into the logon you want to edit, get the image and load it up in your favourite image editor and increase the resolution, then save it. that should work.

I was using 1360x1020 as my default logon background size in my logons(halfway between 1024 and 1600),but now I have a widescreen monitor at 1920x1200 and they still look ok...they just stretch a bit.(splash style logons dont distort the images as badly as a wall displayed at the wrong rez)

Future logons will be wide and standard with widescreen probably being 1650(halfway between 1280 and 1920)

Like these guys said,you can easily replace the background in a splash style logon with a full 1920x1200 image(convert to bitmap from a widescreen wallpaper)  

Shameless self promotion

My logons weren't created in logonstudio. In fact opening them for editing with LS will screw them up

Give me 5 minutes to find some links.... I have just what you need!

Do NOT use the editor on splash logons...just open the folder and replace Bitmap_100.

What he said ^