Love the beach, the runoff/meltwater (?), and aurora reflected!
For your image, get XnView (free, multi-platform), load your original TIFF output (or if you're outputting EXR and tonemapping or something, then save as 8 bit TIFF from there and load in XnView), and then use Export function on the File menu with the JPEG options. You can get very high quality output with good compression (reasonably file size) using the right settings there. I recommend:
DCT Method: Float (best but slowest)
Remove All Metadata: checked (yes)
Smoothing Factor: 0
SubSampling Factor: 1x1, 1x1, 1x1 (best quality)
Progressive: checked (enabled) - this one is fine either way, it only saves a small amount of file size
Optimize Huffman Table: checked (enabled)
Then try Quality slider setting of 85-95, check the preview above for quality (right-side is output, left-side is original, compare the two, make sure you zoom to 100% first though).
You can also use Photoshop's Save for Web feature, but XnView is free if you don't have it.
- Oshyan