>> I don't know if the patch to vertically center line content
>> recently sent here by Jesse, could improve the look to align
>> the button image to the center of the text line in tabs.
>
> The image used should be centered to begin with.  You could also try
> playing with :ascent property of images.

Thanks for the hint.  Using :ascent property with the center value
on images solved the issue, so now tabs are more nice-looking: