With webmail, I don't believe you can. To add photos inline, you have to send the email in rich text (html). The html editor in standard webmail is broken and obsolete. I don't see a way to insert a picture inline using the enhanced webmail rich text editor. If I'm wrong, maybe someone else can clue us in.
With a 3rd party client like Thunderbird, it's easy. Even the Windows 10 built-in email app will do it. Setup your Cox account as imap, that won't effect your webmail usage, in the client of your choice. Create new mail, make sure you select rich text as format, you will have an insert button which will allow you to insert a photo inline with the text. Here's a sample from Windows 10 Mail: