PhotoNinja

Pick Erase or Draw, then a shape: Brush to drag freehand, Smooth Curve — click your two end points first (green = start, pink = end), then add more points to bend the curve between them, and press "Erase/Draw curve" to apply it with a tapered, ellipse-style tip — or Arc: set a Radius, then click once to secure the circle's centre (shown as a red dot), click again to secure the arc's start point, move the mouse to set how far it sweeps, and click a third time to secure the end point and apply it — an uneven, ballpoint-pen-style stroke that tapers to a fine point at both ends. Use Back to undo the last secured point.  In Draw mode, pick a colour or use Eyedropper to sample one straight from the image.  Use Zoom and the on-image arrows (the small panel shows where you are on the full image) to work pixel by pixel. Editing happens at the image's full original resolution, so the saved PNG stays pixel-perfect.