Watermarking with a ttf file:
You can upload a ttf font file into administrator/components/com_igallery/fonts/font.ttf, then insert the text you want to write with into the profile’s settings -- general tab. Please check the font file is named font.ttf You can use any ttf file, see websites like https://www.dafont.com for free ttf files.
Watermarking with an image:
For watermarking with an image, these settings work best:
This is in the Profile General tab, I have uploaded a PNG8 with an alpha transparancy of about 30.
These are my image editor settings:
You can also use a png 24 with a transparancy