To edit an image, double-click on it.
The image editing capabilities are available thanks to Core Image filters.
The list of filters is organized into categories (color, blur, etc.). Some filters have adjustable parameters.
When you select a filter or change any of the parameters, the image in the preview area changes in real time.
Some filters let you modify the image using the mouse. This is indicated by a circular icon (crosshair) next to the name. Select such a filter, place the mouse cursor over the image, and move the mouse while holding down the left mouse button. The image will change depending on the crosshair position.
To change the image size, click the Size button and set new image dimensions.