If gThumb is not listed go to the bottom (lower third) option, "File Types and Programs" and select "Go There", which will bring up a new drop-down page called: Edit file type. In the bottom option, under "Actions", select "Browse" which will let you select gThumb, once you've navigated the directories. It will start in the users directory, i.e., /home/user/. You need to go up two levels to / and then select /usr/bin and then slide the right-side slider to get you close (alphabetically) and then the up/down arrows provide finer control and slower scrolling. You will find it listed here as "gthumb" (no caps for this binary file).
Once you have opened one photo with gThumb, Select "View" (3rd item below the top banner) and you get a drop-down menu. The 3rd item is the slide show. You can adjust the time each picture is up by clicking on "Edit", select the "Preferences" choice at the bottom, and the the right hand tab (Slide Show) in the gThumb Preferences. You can select full or partial screen as well as changing the time each item is shown.
Start by clicking on the "K-on-a-gear" (mepis) or either a red hat or a gnome "footprint" in the lower lefthand corner click on it. This opens the menu; go up the menu and select Graphics (mepis) and then select Kuickshow from the items offered. This gives you kuickshow with all of the directories (folders) under /home/user to choose from. We'll move to the folder that holds your pictures AFTER checking the slide show settings.
Select "Settings" and it's one option, "Configure KuickShow". The 3rd tab has the slideshow settings. After you've adjusted them to suit yourself, look at the Browser Shortcuts. Start slideshow is F2; quit is Control+Q
Try it and see what happens.