Method 1 - this works for any image on Twitter - even if it's not your image.
1. Look for the 'More' link at the bottom of the tweet
2. From the 'More' menu select 'Embed tweet'.
Method 2 - This also works for any tweet but if you have many followers, the tweet may be difficult to find. It's better if the tweet is one of yours.
1. You'll need an account at TweetDeck - https://tweetdeck.twitter.com/
2. In your 'User' column scroll down to your tweet with the picture you want and you'll find the 'pic.twitter…' link there ready to copy and paste.
Method 3. By far the easiest method provided it is your tweet
Delete the tweet! Huh!
Use the menu icon to select 'Delete Tweet' (Don't panic - you are not going to delete it.)
Once you select that the complete text of the tweet will be shown including that elusive 'pic.twitter.com/...' link. You can now copy it and then click the 'Cancel' button.
Incidentally this method is great for collecting the entire text of a tweet for re-use. Sometimes Twitter will shorten a link in it but the text you copy will contain the full link.