Presumably the end marker of your slideshow ended up somewhere behind the end of the actual last image in the show. You just need to move it to the end of the last picture. In order to do this, make sure you're operating in timeline mode (for full control over you slideshow). Use the mode switch on the left-hand side of your arranger to switch to timeline mode. It's the button with a few parallel, horizontal lines on it. Now, use the zoom control on the right-hand side of the screen above the timeline to zoom out, if necessary. You need to look at the timeline above the first/image track. There's a start marker (S) that marks the beginning of your slideshow and an end marker (E) that marks the end. You can set the "E" marker by right-clicking on the timeline (meaning the actual timeline that shows the time positions). Make sure to align it with the end of your last image/object in the slideshow again. Done!