@somiaj, got the idea and did bit of google search.
@rasat, this with Pixmap for each desk:
*FvwmPager: DeskPixmap 0 $[CORE_DIR]/Desker/web.png
*FvwmPager: DeskPixmap 1 $[CORE_DIR]/Desker/email.png
It fills mini-window but not label area.
This is better, I copied from topic FvwmPager DeskColor/DeskPixmap
Colorset 100 Pixmap $[CORE_DIR]/Desker/web.png
Colorset 101 Pixmap $[CORE_DIR]/Desker/email.png
Colorset [desk no] [colorset no]
*FvwmPager: Colorset 0 100
*FvwmPager: Colorset 1 101
Today was a learning day. 
@rasat, this makes your Desker pager great, KISe 1.4.1 from GitHub 