Watermark not working

Hi there, I have a brand new website for digital online sales, with the NextGallery plug in . As I am completely new to this world I am stuck now for over a week because I simply can not get the watermark working on my images. It’s either massive, covering the whole image or there is none! I am not sure where to make the alterations, I tried both with the WordPress settings and with the NextGen plug in.I want it to cover about 1/5 of the image and be transparent. I figured all settings and none of them are working. Please help. Many thanks for suggestions