Hiya I found your problem it's a really easy fix though I just tested it in TuneLab and it works perfectly now
(extra notes
-so for context the name of the utau has to be the same as the folder name so openutau and tunelab can find it but if you want to change the name of the utau just change the name in "name=___" and then the folder needs to be the same name too
-the author name doesn't matter
-just find an image you want then put the whole name of the image file)
what you need to do is create a text file called character.txt
and then paste this in the character.txt
name=Zeta +mellow
author=NUFAloids
image=Zeta +mellow.bmp