Final Lossy Compression JPG Image
You can see signs of artifacts which is especially noticeable in the sky shown as coarse boundary box or cluster of pixels. This is caused by a lossy compression algorithm which discards data to reduce the size of the file.
The following image is the result of a JPG image saved 10 times:
40% resize, 60% compression
10% resize, 50% compression
60% resize, 50% compression
Final for web
JPG is good for photos and images with many gradations of color. However some edge detail may look fuzzy.
16 colors, 60% dither
GIF is best for line image with solid colors, does not do well with gradients. However it does support transparencies.
PNG format give you the best of both JPG and GIF because it supports full alpha transparency and is lossless compression. Good for line image with gradients.