How to include .png image as signature to an email using MIMEMultipart python script

I'm composing an email with MIMEMultipart module of Python 3.7 and try to send it over an "smtp.office365.com" server. I would like to complete the following piece of code adding an email signature composed by an image (.png) and some plain text.

I tried different ways but I'm still stuck on this problem. Can you help me to add the right piece of code for sign the email with a .png image and some plain text? Thank you very much!!

Here is the code I wrote so far:

from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from email.mime.base import MIMEBase
from email.mime.image import MIMEImage
from email import encoders

sender='sender@hotmail.it'
reciever='reciever@gmail.com'

#--create a multi-part message object
message = MIMEMultipart()

#--filling up the header
message['From'] = sender
message['Subject'] = 'Hello!'

#body = some HTML text...

#---retrieve and attach the body from the draft email to the new message object
b = MIMEText(body,'html')
message.attach(b)

#---open the attachment file
filename = "attach.pdf"
attachment = open("C:/Users/attach.pdf",'rb')

#---attach the file to the message object 
p = MIMEBase('application','octet-stream')
p.set_payload((attachment).read())
encoders.encode_base64(p)
p.add_header('Content-Disposition',"attachment; filename= %s" % filename)
message.attach(p)

#---convert the message object to string
text = message.as_string()

#--------------------------------------
#-------------SENDING------------------
#--------------------------------------
import smtplib

SMTPserver = 'smtp.office365.com'

USERNAME = sender
PASSWORD = password

#--start server connection
server = smtplib.SMTP(SMTPserver,'587')
server.starttls()
server.login(USERNAME, PASSWORD)
server.sendmail(sender,reciever,text)
server.quit()