It's not possible to embed an image in HTML. All references to an image must either be the direct URL to the image: http://www.cdrlabs.com/images/forums.gif
or using an image tag:
The problem you are experiencing is that when you save the file in FrontPage, it most likely generates the src="" portion. When you upload the image to Geocities, you are not placing it in the proper location.
For example, on your computer, the image tag may look like:
But up on Geocities, the image tag may need to be in the form:
You will need to make the directory structure match somehow, so that the webpage can find the image. Easiest way to do it on hosts like Geocities is to use relative URLs rather than absolute ones (like the examples I posted above)
There has to be a way to upload an image, because it is not possible to embed a picture in HTML. FTP access is different than uploading an image through the web browser. There should be a section for you to upload files (similar to the way you uploaded the HTML file) through your web browser.