1. A computer implemented method of compressing an image using one of a plurality of compression methods, said method comprising the steps of: analysing a first portion of the image to determine a compressibility score for the whole of the image; comparing the compressibility score for the whole image to a plurality of thresholds, each of the plurality of thresholds being for a candidate compression method from the plurality of compression methods, wherein at least one of the plurality of thresholds is dependent upon an amount of a portion of the image which has been analysed; if the compressibility score for the whole image satisfies a first threshold of the plurality of thresholds, selecting the candidate compression method to compress the whole of the image; if the compressibility score for the whole image satisfies a second threshold of the plurality of thresholds, rejecting the candidate compression method; and if the compressibility score for the whole image satisfies none of the plurality of thresholds, analysing a second portion of the image to update the compressibility score for the whole image to thereby determine if one of the plurality of thresholds is satisfied.