Is the homepage image really a 5mb file? Do yourself and viewers a favor and resize them down to about 400k. They will display fine.
Images displayed on the Web above 72 dpi resolution are pointless, so adjust your images. They will load faster and look just as well.
File format should be jpg or gif. If you have a copy of Photoshop, the best tool in the package is "Save for Web".
If you need a great FREE image program try this one out...
http://www.irfanview.com/
Did I mention that it's FREE??????