This is the normal way: - be sure you are composing in HTML format. You should see a toolbar in compose window that includes the 'insert' option. This will display the photo within the email. If the recipient receives in HTML, the photo will appear within the message, but if the recipient receives plain text messages, the photo will appear as an attachment.

ご返信いただき誠にありがとうございます。

すべての受信者が、写真をメッセージ内に表示できる方法はありませんか？ どうしても添付として表示させたくないです。

No. This is not just Thunderbird, it's all email. The recipient must be using HTML to see HTML messages. Most people do, but some are afraid of scripts and viruses and hackers, all of which are possible when accepting HTML. Thunderbird offers protection against that, but many don't know it, or use other email clients.