Click on the Pictures tab, and then click the Add>>From File button.
Navigate to the folder where your pictures are stored, highlight all you wish to display, and click add.
Arrange the photos in order by highlighting the name of the photo and clicking the move up or move down button.
Click on the name of each photo and type a caption in the caption box.
Click on the name of each photo and change the width (for horizontal pics) or height (for vertical pics) to 150. Make sure the "Keep aspect ratio" option is checked. The other dimension will update automatically.
When done, click ok. To edit the gallery, simply double-click the gallery display on the page.