To resize an Image/Layer:
Click on the "Image->>Resize" menu item, then you will see a dialog form asking for the image size in pixels. If you want the the width and height are bounded as original image, you need to check the bounds checkbox. After you click "ok" button. Then you will see a resized image.
Alternatively, you can use Zoom in and Zoom out button on toolbar to resize the image without losing the resolution.

To rotate an image/Layer:
Click on the rotation button (after zoom out button) dropdown mark on the toolbar and select any angle and click, then you see a rotated image. To specify a particular angle you can select the last choice "Specify". Then you will angle adjust form. with a trace bar for you to specify a exact angle. After you click on "ok", you will get a rotated image as you expected.

To flip an image/Layer:
Click on the flip button (after rotation button) dropdown mark on the toolbar and select horizontally or vertically flip. Then you will see a flipped image.

To adjust the color of an image/Layer:
Click on "Image->>Adjust color" menu item. Then you will see a color adjust and balance control panel. With the trace bar you can adjust contrast, gramma, alpha (transparency) and even three RGB components specifically. As you adjust the preview panel show the exact effect on the original image.

To get an grayscale/negative image/Layer:
Click on "Image->>Grayscale/negative" menu item. Then you will get a grayscale or negative image from original one.

To apply fileters to image/Layer:
Click on "Filter->>..." menu item. Then you will get filtered image/layer.