Julius allows you to easily replace a gray sky with a beautiful blue one right on your iPhone. To change the sky in a photo, click the sky button on the edit photo screen:

Then click the desired new sky:

Then, if needed, use the options to refine the sky replacement:

Threshold adjusts how strongly the sky area is replaced, slide to the right to replace more of the sky area.

Horizon sets the vertical location of the bottom of the sky.

Erase Sky lets you paint on the image to remove parts of the sky you don't want. You can pinch to zoom in for more accuracy.

Add to Sky lets you paint areas to add to the sky. Pinch to zoom in for more accuracy.

Move Sky allows you to adjust the zoom and pan of the sky. Pinch and pan on the sky to move the sky. You can use this to vary the sky from image to image so they aren't all the same.

Opacity lets you change how strongly the new sky overlays the original image.