Press F1 within SED to open the handbook/help
Then search for decal

You can place a sprite like this:

ent_create("blood.bmp", player.x, act_blood);
but of cause you have to adjust the position (x,y,z) and/or the angle (pan,tilt,roll).
Furtheron you have to script the scaling: scale_x, scale_y

Maybe you should start with the tutorials to get an idea of entites and their possibilities...