Extension question

[Trenloe]

I can't find yet how resize the image to fit in the whole fantasy grounds background.

It's controlled by the <bounds> entry of the desktopdecalpanel in CoreRPG desktop\desktop_panels.xml

These bounds keep the maximum size the decal can be to 100 pixels inside of the desktop boundaries (top, bottom, left & right), and the decal will be resized to fit within these maximums as the FG desktop is resized. Plus the decal size on the won't go above the actual size of the decal image, but it will be reduced if the desktop is smaller than the image size (plus the <bounds> setting).

[damned]

you need to remember that every at the table could have a different screen size and screen proportions to you.
to make it cover the whole screen you cant use a decal you need to use a tiled wall paper.
it uses a 9 slice layout -

top left - fixed size
top - tiles horizontally
top right - fixed size
left - tiles vertically
middle - tiles both directions
right - tiles vertically
bottom left - fixed size
bottom - tiles horizontally
bottom right - fixed size

the code looks something like this:

<!-- Desktop background -->
<framedef name="desktop">
<bitmap file="graphics/frames/desktop.jpg" />
<topleft rect="0,0,300,300" />
<top rect="300,0,400,300" />
<topright rect="700,0,300,300" />
<left rect="0,300,300,200" />
<middle rect="300,300,300,200" />
<right rect="700,300,300,200" />
<bottomleft rect="0,500,300,300" />
<bottom rect="300,500,400,300" />
<bottomright rect="700,500,300,300" />
</framedef>
<framedef name="shortcuts">
<bitmap file="graphics/frames/sidebar.png" />
</framedef>

and you can find some basic samples with the basic structure on https://www.fg-con.com/free-downloads but they use only the middle definition - you would use the above code to create a full screen image.
however after all that you might notice that the middle tiles so its generally not going to work with what you are wanting to do here. You would pick a uniform background colour and fade the edges of your decal or a few other variations.
attached is a really basic example.