Problems with images in Outlook Express 6

Hi there folks, I am using Outlook Express 6 on Windows XP. I am trying to add a signature pic to my mail and I have made an html file with the image in it but I have noticed that I cannot embed images in my mail, when sending it says that one or more images cannot be found, send mail anyway? If I send the mail the picture does not display, it just shows up as a little block with an x in it. I have unclicked the option in security settings to block images in sent mail. The funny thing is that some images can be seen whereas others can't. Are there any limitations on images that you embed? The image size is not the issue as I have been able to add bigger pictures.

Any help would be great thank you
HasH

 

I have solved my problem by just exporting the image as a gif instead of a jpeg but would still be interested to know why it won't work as a jpeg