You can mark the area you want to keep (the crop box) on the picture's preview and use the anchors to change the size. You can also move the crop box to bring a detail into the foreground: click on it, hold down the left button of the mouse and drag it to a new position.
You don't have to confirm the crop: the area outside the crop box (darker than the original) will not be displayed. If you change your mind and want to undo the crop box, just click on the area outside it to remove the box and cancel your selection.
The following options are shown in the window, next to the picture:
|▪||Crop: this option shows the values (in pixels) of the X-coordinate and Y-coordinate that correspond to the upper left apex of the selected area. It also gives the Width and Height of the area. You can change the size of the selected area by changing the values shown here (as an alternative to pulling on the anchors of the crop box).|
You can also define the crop box and maintain the correct width/height ratio with the Ratio option.
|▪||Mirror: you can flip the picture horizontally or vertically.|
|▪||Rotation: you can rotate the picture clockwise by 0°, 90°, 180° or 270°.|