To remove stock photos included with your site design in Showit, simply click on the photo you wish to remove, then hit the delete button on your keyboard to remove the photo.
To add your own photo in it's place, make sure you are on the canvas you want to add the photo to. You can check by making sure the name of the canvas you wish to add the photo to is highlighted in blue in the left sidebar.
*Note* This process is for adding a single photo to a canvas or page, for adding photos to a gallery read here.
Click the media library icon located in the bottom middle section of the Showit builder, it's the icon on the right in the row of three.
Once inside the media gallery, click to select the photo you wish to add, then click the add to page button.
Then you are all set, size your image how you wish it to appear in the staging area, and you've done it!