I assure you that some careful reading and exploration will give you the answer you're looking for
My question is if there is a specific extension to do it, perhaps a funtion that would open a window to choose a photo from.
There are basically two directions you can go. You could use get_open_filename(), which opens a dialogue window and allows the user to choose a file from anywhere the file system. However it's pretty gross looking and it's windows only. Additionally I believe you can only grab one file at a time.
Your other option would be to use the file system functions to explore the working directory and see what, if any, image files are present. You'd then build a custom system to produce an in-game "window" for the user to navigate and select the files they want. Or you could iterate through them using file_find_first() and file_find_next() and load them automatically.
Either way you're constrained by GM being Sandboxed. With the exception of get_open_filename() on the windows platform, the image files have to be located either in the game bundle or in the save area. Getting your custom images into one of those spots means either bundling them inside of "included files" or having the user manually move image files into the working directory where GameMaker can fine them.