This post explains in more detail how to find the list of attachments you uploaded previously, and how to determine the URL of your attachments so you can use them in any additional posts where the image would be appropriate. The system doesn't allow for you to upload the same image more than once, so this is the best method to use when the same image is needed in multiple posts.
Go to your User Control Panel ("User CP" in the menu bar above).
(You can barely see it in the upper left corner of this first image.)
Click the "Attachments" link at the bottom of the left menu:
Scroll through, and find the file(s)/photo(s) you are looking for (there may be multiple pages).
Clicking the file name (black arrow) will open the file in a new window (or download it, for certain file types).
Clicking the "Thread" link (blue arrow) will open the first post of the thread the files were initially uploaded to.
Clicking the "Post" link (red arrow) will open the specific post the files were uploaded to.
Use whatever method necessary, to get the URL for the file. The exact methods, and success rate of the following will depend on your browser:
You can
right click the file name, and select the equivalent of "Copy link address", or "Copy URL", etc.
You can left click on the file name, and open the file in a new window. At that point, it is easy to copy the URL from the address bar of your browser.
You can click the "Thread" or "Post" links, to display the files in their original context. At that point, the
right click method can be used again (on the photo itself, or on the file name in the 'attachments' section of the post); or you can click on the file name in the 'attachments' section of the post, and copy the URL when it opens in a new window.
(I simply right click on the file name, when looking at the list of attachments, and select "Copy link address as..." {Google Chrome}; but many methods work.)