Sign into Twitter. A timeline will appear with "tweets" of those whom you follow, with the most recent tweets at the top of the screen. When you see the tweet to which you want to reply, simply hit the "reply" key underneath the tweet. A new screen will appear with that user's Twitter name or "handle." Scroll over and copy the name.
Log into Twitpic using your Twitter username and password. After logging in, a screen will appear, asking to allow Twitpic permission to connect with Twitter. Click on "allow."
Click on "Okay, let's get started" on new screen.
Choose an image. A new Web page will appear with three steps. The first reads "choose an image or video to upload." Below is a button that says "Browse." Click on that button. A window will pop up, giving you a view of the documents, photos and programs on your computer. Select the photo you wish to upload to Twitter, and click "OK." As an aside, all photos uploaded to Twitter must be in a GIF, JPG or PNG format and no larger than 10MB.
Add a reply. Scroll down on the page to step 2. It reads "Add a message." Since you are posting a reply to another Twitter user, paste the handle you copied in step 1 here. This will ensure this particular Twitter user will receive the photo. You can also add a message in the screen if you wish.
Post to Twitter. Step No. 3 reads "Post Options," and consists of two boxes underneath that read "Post to Twitter Account" and "Include Location Data (if available)." Click to place a checkmark next to "Post to Twitter Account," then hit the "Upload" button. This will place the photo in your Twitter timeline, as well as send a reply to a specific user.