How to represent an image as length of Gif compressed file?

I need to measure visual complexity of some images. For quantification of visual complexity Donderi has suggested the use of compression techniques su开发者_StackOverflow社区ch as Gif.

Donderi argues that when a picture is compressed the string of numbers that represent the organisation of that picture is a measure of its information content. So, data compression algorithms might provide a practical way for defining the complexity of a bit string – and therefore of a visual form encoded as a bit string. (Donderi, D. C. (2006). Visual complexity: A review. Psychological Bulletin, 132, 73–97)

My problem is that I know very little about programming and I don't know how to get to the number that will represent picture complexity.

Thanks for any ideas!