。
She had climbed up the ladder and was painting.
Literal
She [topic-は] ladder [onto-に] climb-up-and paint [object-を] was-applying.
ペンキ ('paint') is one of Japanese's older Dutch loanwords (from Dutch pek), dating back to Edo-period contact with Dutch traders at Dejima. Other survivors from that era include コップ (cup), ガラス (glass), ランドセル (school satchel, from ransel), and ビール (beer). The verb 塗る is the right choice for paint, lacquer, ointment, butter — anything spread on a surface. The combination ~ていた (past progressive) here gives the painting an ongoing past quality, as if the speaker is describing a scene already in motion when they noticed it.