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)

After I found out my original answer doesn't work for most images, I did some research and constructed a working solution. Unfortunately, it has a few drawbacks:

• It may not work for all apps (it does work for Discord, though).
• It can't be used to copy images from memory, only from an existing file.
• It is definitely not cross-platform (I doubt it works on older versions of Windows, even. It seems to work fine on Windows 10, at least).

The solution utilizes pywin32 and is as follows:

import os
import win32clipboard as clp

file_path = 'test.png'

clp.OpenClipboard()
clp.EmptyClipboard()

# This works for Discord, but not for Paint.NET:
wide_path = os.path.abspath(file_path).encode('utf-16-le') + b'\0'
clp.SetClipboardData(clp.RegisterClipboardFormat('FileNameW'), wide_path)

# This works for Paint.NET, but not for Discord:
file = open(file_path, 'rb')
clp.SetClipboardData(clp.RegisterClipboardFormat('image/png'), file.read())

clp.CloseClipboard()

I would just use Pillow:

from PIL import ImageGrab
im = ImageGrab.grabclipboard()
im.save('somefile.png','PNG')

scrot + xclip

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

scrot '/tmp/%F_%T_$wx$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_$wx$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_$wx$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.

$wx$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)