Core of the problem

PIL has no native way of opening an image in full screen. And it makes sense that it can’t. What PIL does is it simply opens your file in the default .bmp file viewing program (commonly, Windows Photos on Windows [although this is Windows version dependent]). In order for it to open that program in full screen, PIL would need to know what arguments to send the program. There is no standard syntax for that. Thus, it is impossible.

But, that doesn’t mean that there isn’t a solution to opening images in fullscreen. By using a native library in Python, Tkinter, we can create our own window that displays in fullscreen which shows an image.

Compatibility

In order to avoid being system reliant (calling .dll and .exe files directly). This can be accomplished with Tkinter. Tkinter is a display library. This code will work perfectly on any computer that runs Python 2 or 3.

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Our function

import sys
if sys.version_info[0] == 2:  # the tkinter library changed it's name from Python 2 to 3.
    import Tkinter
    tkinter = Tkinter #I decided to use a library reference to avoid potential naming conflicts with people's programs.
else:
    import tkinter
from PIL import Image, ImageTk

def showPIL(pilImage):
    root = tkinter.Tk()
    w, h = root.winfo_screenwidth(), root.winfo_screenheight()
    root.overrideredirect(1)
    root.geometry("%dx%d+0+0" % (w, h))
    root.focus_set()    
    root.bind("<Escape>", lambda e: (e.widget.withdraw(), e.widget.quit()))
    canvas = tkinter.Canvas(root,width=w,height=h)
    canvas.pack()
    canvas.configure(background='black')
    imgWidth, imgHeight = pilImage.size
    if imgWidth > w or imgHeight > h:
        ratio = min(w/imgWidth, h/imgHeight)
        imgWidth = int(imgWidth*ratio)
        imgHeight = int(imgHeight*ratio)
        pilImage = pilImage.resize((imgWidth,imgHeight), Image.ANTIALIAS)
    image = ImageTk.PhotoImage(pilImage)
    imagesprite = canvas.create_image(w/2,h/2,image=image)
    root.mainloop()

Usage

pilImage = Image.open("colagem3.png")
showPIL(pilImage)

Output

It creates a fullscreen window with your image centered on a black canvas. If need be, your image will be resized. Here’s a visual of it:

Note: use escape to close fullscreen