@Wixen, you're using Linux Lite, right? (And the OP is using Linux Mint.) You both should have a screensaver built-in, including for slideshows. Click your Menu button in the lower left corner, type in screensaver and click on it to bring up its Preferences. You may need to first enable the screensaver with the slide switch at the top of the Preferences window. Then scroll down the list of Themes on the left and click on Slideshow. Finally, click the icon below the Themes (with 4 horizontal lines) so you can set the Slideshow specific preferences, like the Location of your photos, and whether to randomize or stretch the images. I don't see a setting to control how long each photo stays before switching to the next one. Be sure to Activate the screensaver and set the idle time before it engages.
You may also like to investigate having your desktop wallpaper change at set intervals to give a similar slideshow effect while you're using the computer, rather than when it is sleeping. If so, right-click on an open area of your desktop and select Desktop Settings. This shows the built-in wallpaper you can select, but below the pictures you can change the Folder to where your own photos are stored. Then near the bottom you can tick the box to "Change the background" and set the interval and whether you want the photos changed randomly.
These settings are available in many Ubuntu-based distros, I think. The above details were found in Linux Lite 5.4. Linux Mint (20.1 MATE Edition) has a similar screensaver, but there are fewer
options for both screensaver and desktop backgrounds. The slideshow screensaver in Mint (MATE) is called "Pictures folder" and photos would need to be specifically moved there to work (~/Pictures). The Mint screensaver seems to have many more basic screensavers included than Lite.