the mechanism behind ImagePosition in mathematica 12

I would like to write an image pattern recognition method in mathematica 12 to find the position of a custom pattern(e.g. an icon from the webpage, not necessarily in the category given by mathematica). Does mathematica12 offer the possibility to let users define their own categories? If not, I would like to know the outline of the algorithm behind the ImagePosition[] method so I can write my own.

Thank you for your advice. Best.