I hope that you're using PhotoStory and not Photoshop.
Make sure that you're in Timeline mode and not StoryBoard. Grab the lower left or lower right handle of the image object and drag it left or right.
Or, right-click on the image object and select View Duration. Change the duration in the dialogue box by dragging the slider or typing values into the m:s:fr box. "fr" means frames.
The default duration of any new image is 7 seconds. You can change the default under the Program settings, File, Settings, Programs, Options tab, to what you want. I have mine set at 4 seconds.
You can also trim an image or video clip using the scissors or selections under the scissors dropdown. Click on the down arrow to see the options and not the shortcuts as these will be most useful to you. So, you can place the playback marker exactly where you want the image to end, select the image, shortcut U, and the right side will be cut out leaving a blank. Click on the button to the right of the arrow to be able to select all objects to the right, and drag the one to the right of the hole to the left to close the gap. Alternatively, if this button is set before doing the U, then the gap will be closed automatically. Do some tests to see how this works.
Unfortunately, PhotoStory does not have a setting for just doing this on a track. Movie Edit Pro Plus/Premium has this possibility and much more.