The question relates more to your email system than to Firefox.

As general comments

• files and images are likely to be sent as attachments
• there may be options as to how they display, hotmail for instance is able to display attatched images in firefox as a slideshow within the email
• sending links rather than attached files is an alternative if the .jpeg is on the web
• Hotmail, also allows whole Office Files to be inserted. (That includes word processor documents containing images)

If it is Hotmail and using windows XP you have tried

• insert | Photos -> web images
  left clicking on a displayed image will insert it at the cursor position in the e-mail

Note Hotmail has its own help information, and a help forum {clickable link)

Thank you John99 for a prompt reply.

However, this hasn't helped me, may be I could not explain my self. I tried Hotmail as you have suggested, but it doesn't solve my problem. I would like to insert a jpg file from those saved on my hard disk into the e mail. I have learnt how to send jpg and other files as attachments and now I would like to learn how to insert them inside the e mail.

I assume that you are normally using Hotmail.

If so either insert the jpg as an attachment, Hotmail will then offer to display the pictures in a slideshow in the final email.

Or Make a WP document set out as you like and send that. I am not aware of any other easy method of inserting pictures within hotmail emails, other than as stated in my last post, but you may get a faster and more acurate reply from the Hotmail forum.