Don't see images on my buttons if I create them with a loop - image

Question

I'm trying to make a program in Python 3.3.0 to train with Tkinter, but when I try to put images on buttons which are created in a loop, I obtain a few buttons that don't work (I can't click on them and they don't have images), and the last one is working and with the image on it. Here there's the code:
elenco = [immagine1, immagine2, immagine3, immagine 4]
class secondWindow:
def __init__(self):
self.secondWindow = Tk()
self.secondWindow.geometry ('500x650+400+30')
class mainWindow:
def __init__(self):
self.mainWindow = Tk()
self.mainWindow.geometry ('1100x650+100+10')
self.mainWindow.title('MainWindow')
def Buttons(self, stringa):
i = 0
for _ in elenco:
if stringa in _.lower():
j = int(i/10)
self.IM = PIL.Image.open (_ + ".jpg")
self.II = PIL.ImageTk.PhotoImage (self.IM)
self.button = Button(text = _, compound = 'top', image = self.II, command = secondWindow).grid(row = j, column = i-j*10)
i += 1
def mainEnter (self):
testoEntry = StringVar()
self.mainEntry = Entry(self.mainWindow, textvariable = testoEntry).place (x = 900, y = 20)
def search ():
testoEntry2 = testoEntry.get()
if testoEntry2 == "":
pass
else:
testoEntry2 = testoEntry2.lower()
mainWindow.Buttons(self, testoEntry2)
self.button4Entry = Button (self.mainWindow, text = 'search', command = search).place (x = 1050, y = 17)
MW = mainWindow()
MW.mainEnter()
mainloop()
If I try to create buttons in a loop without images, they work:
def Buttons(self, stringa):
i = 0
for _ in elenco:
if stringa in _.lower():
j = int(i/10)
self.button = Button(text = _, command = secondWindow).grid(row = j, column = i-j*10)
i += 1
And if I try to create a button with an image but not in a loop, it works too:
im = PIL.Image.open("immagine1.jpg")
ge = PIL.ImageTk.PhotoImage (im)
butt = Button(text = 'immagine', compound = 'top', image = ge, command = secondWindow).grid(row = 0, column = 0)

Answer 1

Let's say you have images named "image-i.png", i=0,..,9.
When I execute the following code (with python 3.5) I obtain ten working buttons with image and text:
from tkinter import Tk, Button, PhotoImage
root = Tk()
images = []
buttons = []
for i in range(10):
im = PhotoImage(master=root, file="image-%i.png" % i)
b = Button(root, text="Button %i" % i, image=im, compound="left",
command=lambda x=i: print(x))
b.grid(row=i)
images.append(im)
buttons.append(b)
root.mainloop()