Photo mode — P to leave, WASD/arrows pan, scroll zoom
Chest
Click an item (or drag it) to move the stack across; right click moves one.
Wearing
Head, body, legs, hands, feet and back. Click something wearable below (a steampack goes on your back) to put it on; click what you wear to take it off.
Inventory
Click an item to put it in the selected slot of the bar at the bottom, or drag it onto a slot (or hover one and press 1–9); drag from the bar back here to take it off.