Web pages themselves are always pure text, they can't contain pictures,
so when XL converts your sheet to a web page, it stores the picture in a
folder which also has to be uploaded to your site. The web page then
contains a relative link to that folder.
The folder name may depend on version, but it generally has the structure
<filename>_files
where filename is the name under which you saved the web page.
Make sure you either upload the folder at the same level as the web
page, so that the relative link works correctly, or that you instead
edit the link(s) in the web page to point to wherever you put the
picture.
