Do you have music or some other media on a parallel track that's keeping this space from being a true gap in your timeline?
Trim back your music temporarily and you should be able to double-click on the space between your pictures and right-click and select Delete and Close Gap.
Although you also reference Diashow slideshow creator, which has nothing to do with Adobe or Premiere Elements. If you're working on Dia, you'll need to contact support for that program for help. This is an Adobe Premiere Elements forum.
If you are talking about Delete and Close Gap used in Premiere Elements 13 Expert workspace Timeline, please check out the principles in the following to determine if the "Delete and Close Gap" failures described apply to your situation.