Mozilla Thunderbird and Using Clipart

Since we changed from Netscape to Mozilla Thunderbird (version 1.5.0.5), I’m having difficulty using clipart. Images saved previously (e.g., gif, jpg) don’t always work. Sometimes they stay in message, but most of the time not. It is random. Image appears in message when I write it, but does not stay in message when sent. Any ideas why or how to fix?