Hidden or Invisible Layer?

I may have a hidden layer on my web site home page.
at the bottom of the page say 1/5th of the page.
if I place an image on the page and move it down so
that some of it enters into that 1/5th of the page, it looks OK
in my work area, but the thumbnail in my Page and Layers Gallery,
shows a white shadow on the lower part of my image.
if I do a prieview of my page in any browser, again all OK the image is complete.
However if I upload my web site, the bottom portion of my image will be missing?.
I've strained my eyes looking through the MouseOff section in the Page & Layer Gallery
and I can't find anything to solve this problem.
When I right click on the image, and select arrange the options
Bring to Front and Move Forwards are not available to me, why?
I forced a full site upload, but did not solve the problem.
I have updated my web site for this week (which is public) but I would like this
resolved for next week, if possible ....... Thank you in advance