To brighten a dark image with a bright sky, use Levels on the dark areas.

1.  SELECT. Use the Magic Wand to select the sky. If you uncheck Contiguous, you can get all the light values.

Select menu >Inverse the selection to work on the dark parts of the image.

2.  BRIGHTEN. Choose from the Image menu > Adjustments, Levels. 

Select Channel = Red to adjust the brightness distribution of the Red pixels.  At the right side of the graph, slide the white triangle to the left to push the pixels towards white.   Leave the white triangle at where initial pixels are graphed.

Repeat for the Green and Blue Channels.  To further adjust the color balance or brightness, slide the location of the middle grey triangle - this moves where the middle value of the color falls.

3. SAVE.  Save the file as Photoshop, then use Image Menu, Image Size to resize to 400 x 300 Pixels.  File menu > Save for Web: select the 4-up tab to compare quality for photographs, choose a high-quality JPG format. Note that each time a JPG is saved, the quality is degraded.   GIF's save images with 256 or fewer colors without loss, and can show transparency.