Hello there, Pete.
The following information from the iPhoto Help Guide should provide help with modifying your slideshow depending on which option you prefer:
Thanks for reaching out to Apple Support Communities.
When you start the slideshow are any of the slides selected or just the album in the left hand pane?
Try the following:
1 - delete the iPhoto preference file, com.apple.iPhoto.plist, that resides in your
User/Home/Library/ Preferences folder.
2 - delete iPhoto's cache file, Cache.db, that is located in your
User/Home/Library/Caches/com.apple.iPhoto folder (Snow Leopard and Earlier).
or with Lion, Mt. Lion or Mavericks delete the contents the User/Library/Containers/com.apple.iPhoto/
3 - reboot, launch iPhoto and try the slideshow again.
NOTE: In Lion and Mountain Lion the Home/Library folder is now invisible. To make it permanently visible enter the following in the Terminal application window: chflags nohidden ~/Library and press the Return key - 10.7: Un-hide the User Library folder.
If you're running Mavericks, 10.9, go to your Home folder and use the View ➙ Show View Options menu to bring the this window:
where you can check the Show Library Folder checkbox.
I found this massive amount of stuff in the library after following your instructions in how to "show" the library... but i was afraid to eliminate anything....PS the cache folder was empty. Thanks for your help... just a little confused... only been using iPhoto for 15 years! Pete
<E-mail Edited by Host>
Two comments about your post:
1 - don't put your email address in any posts as they can be easily obtained by spammers who can automatically search webpages for email addresses to gleam and use. I've asked the hosts to edit it out of your post.
2 - you're screenshot didn't make it. Try again using the camera buttion above the post field.
If you want or need to change the default format of image files created by screenshots here's how:
Change Screenshot File Type
Copy and Paste the following command in the Terminal window followed by the Return or Enter key:
defaults write com.apple.screencapture type jpg && killall SystemUIServer
Return or Enter key
You don't want to clean the caches frequently. They are there to help apps and the system operate more efficently. Occasionally if there are problems specific caches can be deleted. Too many users have reported problems and then we found they had used Cleam My Mac. I agree with TD that could be a cause.