Upload the image from your hard drive to your website's media library.(In your Dashboard, go to Media>Add New and drag and drop it in).

Then go to your Library and click on the image (it should be the first one shown if you've just uploaded it) to reach the image's Attachment Details page.

On the right you will see the image URL. Copy the full image URL (copy and paste into notepad or word for example, or just hold it in your clipboard memory using the CTRL plus C keyboard keys). It will be something like http://mywebsite.com/wp-content/uploads/date/twitter-followers.png

You can also change the title and Alt text to make them more SEO friendly (eg Title: Twitter followers; Alt tag: my twitter follower count).