Sometimes transperent images are filled in with black, otherwise white. It should always be black since it causes ugly white boxes around the image otherwise as can be seen in attached picture. The green arrow is in both cases generated by Template:Increase, once with a white box and once without. Screenshot is taken from List of countries by Human Development Index. There is also related task [T182128], but I failed to replicate the same effect of math expressions not having a white box sometimes.