A sight picture is what your eye sees when correctly aligned sights are placed onto a target. Sight alignment answers "are the sights lined up with each other", the sight picture answers "where is that alignment pointing". The two are trained in that order: geometry first, target second.
There are three classic sight pictures. Center hold - the top of the front sight cuts the center of the target, and hits arrive at that edge; most modern factory pistols are zeroed for it, and practical shooters treat it as the default. 6 o'clock hold - the front sight touches the target from below, "a pumpkin on a post", with hits arriving above the sight; a bullseye sport classic. Combat hold - the front sight covers the target, hits hide behind the post; fast up close, rough on precision.
With iron sights the eye keeps its focus on the front sight in every one of these pictures: the target stays slightly soft. In dry fire the sight picture is trained after alignment holds steady - first against a blank wall, then on a target with a defined aiming point.