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Each image can be voted on only once by each user; this is tracked by a unique cookie issued to each browser. This can be circumvented by using a new browser or clearing cookies -- the purpose is to show each user which images they've already voted on.
With the new MapMill 2.0 platform, we've made it possible for the Public Lab non-profit to affordably provide hosting for thousands of images, but this comes with certain constraints.
Attempting to upload a large number of images (>100) at the same time may cause slowness on your computer. If you have a lot of images, it may help to use a more powerful computer, or to upload in batches.
Why does this happen? To optimize hosting, image thumbnail generation is performed in your browser, rather than on the server. This means that if you drag a thousand images into the upload form at once, your computer must generate all those thumbnails, which could overload your machine. (This is preferable to overloading the server!) A means of generating thumbnails in batches has been suggested here, but has not yet been implemented.
If you find a bug or experience problems, or if you are interested in helping to improve MapMill (an open source program), please file an issue on Github. If you are unable to use Github, you may contact email@example.com for assistance. Thanks!