Pixlr is a web based image editor, also available as app for Android and iPad/iPhone. Is completely free and is owned by Autodesk (owners of well known AutoCAD) after they have bough from an independent Swedish developer.
This online editor can open common image types such as PNG, JPG, GIF, BMP and PSD or an URL to an image. It supports transparency and includes tools to make adjustments (blur, sharpen, denoise, vignette ..), works with layers and masks, flip, rotate, crop, magic wand, pencil, brushes, red-eye and many more.
Includes a working Window in the left as you probably are used from other image editing applications and a horizontal file menu with more options.