If you want to use renderHeader it could be a problem:

It would be nice to have a prop with custom styles.
Try add styles to the element returned by renderHeader?
I know that I can move the title, but counter is in usual title's place. I would prefer to move counter somewhere and keep title as it is.
@jvaclavik , you can rewrite counter by pass renderIndicator to it.
<ImageViewer renderIndicator={ (currentIndex, allSize) => {
return <div style={...}>{currentIndex} / {allSize}</div>
} }/>
If you want to remove the counter you'll need a function:
renderIndicator={() => null}
renderIndicator={null} (this will not work)
@jvaclavik is true but if you want set renderIndicator you should set styles with position:"absolute", zIndex: 1, top: 0, left: 0 ...
@KA5558 Yes, it's a bit troublesome, but there are more possibilities for doing something special.
Most helpful comment
If you want to remove the counter you'll need a function:
renderIndicator={() => null}renderIndicator={null}(this will not work)