The chapter illustration for Chapter Twelve of Chamber of Secrets shows Harry examining himself in a mirror after using the Polyjuice potion to transform into Goyle. Interestingly, the light above the mirror seems to be some sort of electric light/lamp.
The description of (what is apparently) this scene in the book doesn’t mention the light source:
Harry unlocked his door and stepped in front of the cracked mirror. Goyle stared back at him out of dull, deepset eyes. Harry scratched his ear. So did Goyle.
However, back in Chapter Nine the bathroom is described in more detail:
It was the gloomiest, most depressing bathroom Harry had ever set foot in. Under a large, cracked, and spotted mirror were a row of chipped sinks. The floor was damp and reflected the dull light given off by the stubs of a few candles, burning low in their holders; the wooden doors to the stalls were flaking and scratched and one of them was dangling off its hinges.
It thus seems that the light source in the bathroom was candles. Indeed, throughout the series there are many references to candles and torches as the lighting system in Hogwarts. I don’t believe there are any references to electric lights.
More importantly, we know from Chapter Twenty-Eight of Goblet of Fire that electricity doesn’t even work properly at Hogwarts:
“All those substitutes for magic Muggles use — electricity, computers, and radar, and all those things — they all go haywire around Hogwarts, there’s too much magic in the air. No, Rita’s using magic to eavesdrop, she must be.... If I could just find out what it is... ooh, if it’s illegal, I’ll have her...”
In light of the above, how can this illustration be explained?