There are some sites you can download useful doc from.
E.g. the official modding manual you can get from the shrapnel site
http://www.shrapnelgames.com/Illwinter/d2/6.htm
Check the links in the pinned thread for more.
Some more specific answers:
to 1): you take screenshots ingame with completly black map. (someone made a savegame for that, complete with map etc. Called "unit view" or something, IIRC) You'll only get the 'standard' pose, though. The 2nd (attack) pose you'll have to do by yourself. (or borrow the graphics from another mod...)
to 2): Depends on what grahpics you want to add. The 'gui' has been changed before, don't have a link ready. For units, there's a modding command that lets you assign graphics. Other graphics you can't change.