Q: how you change the speed of pictures displayed on your screensaver

Unfortunately, Apple does not offer an option for the speed at which the photos rotate >  Mac Basics: Display a screen saver when you're not using your Mac - Apple Support

However, you can select how long after you put the Mac to sleep that you want the screen saver to start.

Open System Preferences > Desktop & Screen Saver then click the Start after: pop up menu bottom of that pane.

If yours is an older Apple computer running an OS X prior to Yosemite 10.10.x you may

have other choices in the System Preferences> Desktop and Screen Savers settings...

An example:

A screenshot in Mavericks 10.9.5: There are controls to set duration of rotation to select

times between slideshow images; in older OS X versions, most have control over this.

{And you can choose images from your own folders, set up to be desktop backgrounds.}

However it would appear the controls changed in Yosemite. I've not upgraded newer Mac

to the latest system, so mine shipped (new-old-stock) with Mavericks 10.9 late last year.

Not sure if there would be a third-party app or option to control this system function safely.

In any event...

Good luck & happy computing!