I did copy the code and replace the StringIO with BytesIO and it worked! (with *.jpg and *.png files) Thank you so much!

from io import BytesIO
import win32clipboard
from PIL import Image

def send_to_clipboard(clip_type, data):
    win32clipboard.OpenClipboard()
    win32clipboard.EmptyClipboard()
    win32clipboard.SetClipboardData(clip_type, data)
    win32clipboard.CloseClipboard()

filepath = 'Ico2.png'
image = Image.open(filepath)

output = BytesIO()
image.convert("RGB").save(output, "BMP")
data = output.getvalue()[14:]
output.close()

send_to_clipboard(win32clipboard.CF_DIB, data)

scrot + xclip

You can use scrot with xclip to take a screenshot and copy it to clipboard.

scrot '/tmp/%F_%T_h.png' -e 'xclip -selection clipboard -target image/png -i $f'

It will capture whole of your screen and copy the image to clipboard. If you want to capture current window then use -u flag. For selection of particular area, you can add -s flag. See $ man scrot for more options.

It will store your screenshot in /tmp directory. You can change that directory wherever you want it to get stored. Files from /tmp directory usually gets deleted after each reboot. If you want to immediately remove the stored file, then do something like:

scrot -w '/tmp/%F_%T_h.png' -e 'xclip -selection clipboard -target image/png -i $f && rm $f'

As I read in other comments, you need it for copying a screenshot to the clipboard. I hope this answers your question.

If you just need to copy an already existing image file to clipboard:

cat 2018-06-16-224938_670x730_scrot.png | xclip -selection clipboard -target image/png -i

You can set keyboard shortcuts/keybindings according to your current Desktop Environment/window manager.

Bonus

Explanation of /tmp/%F_%T_h.png:

It's being used as the file name. These are called format specifiers. They are of two type: starting with % or $.

%F     Equivalent to %Y-%m-%d (the ISO 8601 date format).

%T     The time in 24-hour notation (%H:%M:%S).

%F_%T_ will print something like: 2018-06-17_02:52:19_ i.e. your current timestamp. You can customize the format as per your requirements. See $ man strftime for more help.

h are part of the scrot's internal specifiers.

$w   image width
$h   image height

So the final file name will look something like 2018-06-17_02:52:19_1365x384.png.

from cStringIO import StringIO
import win32clipboard
from PIL import Image

def send_to_clipboard(clip_type, data):
    win32clipboard.OpenClipboard()
    win32clipboard.EmptyClipboard()
    win32clipboard.SetClipboardData(clip_type, data)
    win32clipboard.CloseClipboard()

filepath = 'image.jpg'
image = Image.open(filepath)

output = StringIO()
image.convert("RGB").save(output, "BMP")
data = output.getvalue()[14:]
output.close()

send_to_clipboard(win32clipboard.CF_DIB, data)

Question: How to add/get image data in the OS clipboard using Python?

I show only get:
This example is using the built-in Tkinter module to get image data from CLIPBOARD.
Tested only on Linux, but should be a cross-platform solution.

Note: The first paste of the shown 387x388 GIF, takes 4 seconds.

Core point: You have to use a MIME-Type, to requests a image.

.clipboard_get(type='image/png')

Verfied with, 'GIF', 'PNG' and 'JPEG', as source image data, using application, GIMP and PyCharm. With type='image/png' you allways get image data of type 'PNG' if the source app support this.

Reference:

• clippboard_get(type=<string>)

  Retrieve data from the clipboard. Type specifies the form in which the data is to be returned and should be an atom name such as STRING or FILE_NAME. Type defaults on modern X11 systems to UTF8_STRING.

Data format:

0x89 0x50 0x4e 0x47 0xd 0xa 0x1a 0xa 0x0 0x0 0x0 0xd 0x49 0x48 0x44

The data is divided into fields separated by white space; each field is converted to its atom value, and the 32-bit atom value is transmitted instead of the atom name.

After removing the white space and casting with int(<field>, 0):

bytearray(b'\x89PNG\r\n\x1a\n\x00\x00\x00\rIHD...')
<PIL.PngImagePlugin.PngImageFile image mode=P size=387x388 at 0xF555E20C>

Exception: If no selection at all or the source app does not provide 'image/png'.

TclError:CLIPBOARD selection doesn't exist or form "image/png" not defined

import tkinter as tk
from PIL import Image, ImageTk
import io


class App(tk.Tk):
    def __init__(self):
        super().__init__()  # options=(tk.Menu,))
        self.menubar = tk.Menu()
        self.config(menu=self.menubar)
        self.menubar.add_command(label='paste', command=self.on_paste)
        
        self.label = tk.Label(self, text="CLIPBOARD image", font=("David", 18),
                              image='', compound='center')
        self.label.grid(row=0, column=0, sticky='w')

    def on_paste(self):
        self.label.configure(image='')
        self.update_idletasks()
        
        try:
            b = bytearray()
            h = ''
            for c in self.clipboard_get(type='image/png'):
                if c == ' ':
                    try:
                        b.append(int(h, 0))
                    except Exception as e:
                        print('Exception:{}'.format(e))
                    h = ''
                else:
                    h += c

        except tk.TclError as e:
            b = None
            print('TclError:{}'.format(e))
        finally:
            if b is not None:
                with Image.open(io.BytesIO(b)) as img:
                    print('{}'.format(img))
                    self.label.image = ImageTk.PhotoImage(img.resize((100, 100), Image.LANCZOS))
                    self.label.configure(image=self.label.image)

Tested with Python: 3.5 - 'TclVersion': 8.6 'TkVersion': 8.6