picture, sketch, drawing, stroke, mark, divide
畫 centers on creating a visual representation by marking lines: a picture, sketch, or drawing. The sense of dividing or marking boundaries extends naturally from drawing lines that separate areas.
畫 originally depicted a hand holding a brush over a field or boundary, combining the ideas of drawing and dividing. The modern form preserves 聿 (brush) over 田 (field) with a bottom line, reflecting the act of marking out areas.
A brush 聿 draws lines across a field 田, marking boundaries and creating a picture. The bottom stroke 一 completes the frame, like the final line of a sketch.
For ガ, imagine a gallery of pictures: gallery -> ガ, and each 畫 is a framed drawing on the wall.