PHP output pages to MS Word - images not loading

I have several pages in my work program that are sent to MS Word. All of the text loads fine. However, the pages with images are a bit touchy. I have one page that reliably loads the image it has (letterhead image). When the other pages don’t load their images, I load that page and leave it open and that causes the other pages to work properly. When the images don’t load, I get the placeholder red X from Word. If I leave a page loaded in Word, the rest of the documents I run always load all images.

I use this code to force the pages to be sent to Word:

header ( 'Content-Description: File Transfer' );
header ( 'Content-type: application/msword' );
header ( 'Content-Disposition: attachment; filename="' . $filename . '.doc"' );

Is there any reason why either my server isn’t responding with the images sometimes, or could it be a browser setting that I have (my coworker doesn’t seem to have this problem)?

Any ideas would be appreciated.