[chmike]

Another possibility is to add a service to label/tag images.

See if if you could turn it into a game where people would gain karma points by properly labelling/tagging images. Make it peoples choice to participate in it instead of forcing them into it with a captcha.

Your image stack would gain significant value by being labelled and searchable by label.

See https://www.cs.cmu.edu/~biglou/ESP.pdf