背 (Kanji for stature / back of the body)

Onyomi

HAI	I haven't had my coffee yet. Make up your own damn mnemonic. Grr.

Northerners like Norwegians are taller than Japanese because of their long backs - and a back is an organ of the body.

せ	height. (NUBI) You gotta say like 背が高い! or 背が低い! NUBI ★★★★☆

背景(はいけい)

background ★★★☆☆
背 (stature / back of the body) + 景 (scene) = 背景 (background)

background (both in the sense of the background of a photo, and the background context of a story)

背中(せなか)

small of the back ★★☆☆☆ KUNKUN
背 (stature / back of the body) + 中 (middle) = 背中 (small of the back)

背負う(せお＊う)

carry on back ★★☆☆☆
背 (stature / back of the body) + 負 (lose) = 背負う (carry on back)

せお＊う： lit： to carry on one's back. Figuratively, to assume the burden of.