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There is not an option to adjust the slide show speed.
The only way to do it is to edit the registry. Your best
bet may be to simply left click the pause button and
advance or reverse the images using your arrow keys.

If you prefer to edit the registry, read the following:

I would suggest that you carefully read the
following article first before proceeding:

Information About Editing the Registry
http://support.microsoft.com/?kbid=256986#Topic2

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There's a script you can download from the
following site that will increase the timeout
to 10 seconds...scroll down to #81.

http://www.kellys-korner-xp.com/xp_tweaks.htm

Also, the following info was previously posted
by David Shiflet:

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The value of the number is represented in milliseconds
between image changes:
1000 = 1 second, 5000 = 5 seconds, 10000=10 seconds,
etc. To modify my script change the time accordingly.

In order to change the slideshow timing, open regedit
and go to the following key.

[HKEY_CURRENT_USER\Software\Microsoft\Windows\
CurrentVersion\Explorer\ShellImageView]

Create a DWORD named "Timeout", with the value the
number of millseconds between image changes (1000
(Dec) = 1 second, 5000 = 5 seconds)

Note that the time of actually changing the image is not
included, so 1000 =1 second + time to decode the image

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If you open the script from Kelly's site in notepad, you
should be able to use David's instructions to change
the timeout to anything you require.

Good luck....