It comes from something called phosphorescent pigments mixed into the paint of your OSRS Painting.These pigments absorb light when exposed to it and then release it slowly in the dark, creating the glowing effect.

You can charge your glow in the dark art by exposing it to any light source for a few minutes. The best light sources are those that emit ultraviolet or blue light, such as black lights, LED lights, fluorescent lights, etc. You can also use natural light or sunlight. Avoid using incandescent or halogen lights, as they may not provide enough energy for your art to glow.

Depending on how it's charged and the type of light source, the glow can keep up for as long as 12 hours. To get the best shine, put the OSRS artwork where it gets a lot of sunlight during the day. That way, it'll be all set to light up your evening.