One picture can be held for the whole word: on its feet, and staying there without help. Nothing is lying flat, folded down or sitting back. A frame that stays up on a desk is the plainest version of it. Move the same picture to people and you get the rest: one performer standing alone in front of a room, a group of workers who stay on their feet so the meeting stays short, two people facing each other and not walking away, and a person who stands up for what is right when it would be easier to sit still. The word came straight from the verb phrase stand up, so the action never left it.
From the word’s history
The source records stand-up as a deverbal formation from the verb phrase stand up: the two-word action was taken whole and used as an adjective and a noun. That is why every branch of the word still shows someone or something on its feet.
來源資料把 stand-up 記為動詞片語 stand up 的轉品（deverbal）：整個「站起來」的動作被直接拿來當形容詞和名詞用。所以這個字的每一支意思，底下都還看得到那個站著的動作。
Wiktionary, "stand-up"
登入就可以看每一支怎麼連回核心畫面。