The regular search index of Google (using Caffeine) is a "fluent" or quick index, which allows for up to the minute updates. The image search system does not work in the same way. Updates can take up to a month for image search.
It has been noticed by webmasters and designers that images have been included within Google in within a few days, which is a markedly quicker.
A Google spokesperson confirmed changes to Google Image Search which increased the rate at which images are included in the database.