I'll take a wild guess and say that in the self-portrait, Himuro uses his left hand to hold the paintbrush since the image is mirrored.
But in an earlier chapter, Kindaichi ask Himuro for an autograph and he uses his right hand.
I can think of 3 possible explanations for this:
1. Himuro is ambidextrous
2. There are 2 Himuro
3. Somebody is impersonating Himuro after the plane crash mentioned in an earlier chapter