From docs
About the “filetypes” argument
If specified, it should contain a list of items, where each item is
either:
a string containing a filemask # e.g. “*.txt”
a list of strings, where
all of the strings except the last one are filemasks (each beginning
with “.”, such as “.txt” for text files, “*.py” for Python files,
etc.). and the last string contains a filetype description
EXAMPLE:
filetypes = ["*.css", ["*.htm", "*.html", "HTML files"] ]
So corresponding to your usecase, you can write:
file=easygui.fileopenbox(title='Select image', filetypes=[["*.png", "*.jpg", "Image files"]])