I say image compression is not that easy then check this article

 STORAGE FORMATES FOR PICTURE

There are many ways to compress image files, 

but there are two general principles used in this algorithm, that is:

• Redundancy:

In a photo or picture, neighbouring pixel are highly correlated. Lets give a pixel it is very easy to guess the neighbours.

• Irrelevancy

Normally, Our eyes do not accept high resolution pictures and lots of colours. 

The main criteria to compression algorithm and compressed files:

• How much does the image file get compressed?

• How much time does it take to compress the image file?

• How much time it will take to expand ready for viewing?

• Quality will be reduced due to compression or expansion?

• Can the file be exchanged among users?

The above criteria is Very important.

The are two major file Formats to compress or Expand pictures:

• Graphics Interchange Format(GIF).

• Joint Photographic Expert Group(JPEG)

I will briefly describe (GIF) and (JPEG) in next article.