You need to put an image online somewhere to post it in a message (as either an image or link). Your ISP may include webspace with your account, or check out Photobucket. See also;

free web host with hotlinking? (for avatars) - some information may be out of date, as web hosts can change their policies or practices

graphics programs, for editing screen shots

You can also just email me the screenshot (the address is in my profile).