whats the 2 sizes the wallpaper can be on a 7100t

Full screen 240 X 260
To fit and see within the original themes - 240 x 195

For the O2 theme (and perhaps others), make the wallpaper to be 240 x 260 (width x height) with a 75 pixel whitespace area at the top.

The way I do it is to make an image 240 x 185 and then set the canvas level to 240 x 260, moving the actual image to the bottom. Its quite simple. The reason for the whitespace is to allow for your image to fit exactly as it should. The viewable canvas for a background with this theme is 185 pixels with a 75 pixel header space (covering the entire 260 vertical pixels).

Is it 240x185 and not 240x195???

on the O2 theme, yes. make one at 184 and you will see a small one-pixel white line just under the ribbon. extend it to 185 and the line is gone.

240x260 includes both the title bar and the ribbon;
240x195 excludes the title bar but includes the ribbon;
240x185 excludes both the title bar and the ribbon