A short pin with a flat, broad head, used to attach paper or light items to a bulletin board or wall.
I pinned the poster to the wall with a thumbtack.
画鋲を踏んでしまって痛かった。
I stepped on a thumbtack and it hurt.
Standard kanji spelling.
Mixed kanji-kana spelling, common when the second kanji is considered difficult.
画 (picture) + 鋲 (rivet, tack). The kanji 鋲 is a kokuji (Japanese-created character) combining 金 (metal) and 兵 (soldier/weapon), suggesting a metal fastener.