I don't actually think it's very similar. merlyn was trying to find similar images (differing by resolution, format, etc, but otherwise more or less identical) whereas moof1138 is trying to find similar objects - the images he will be comparing will all be lit differently and be photographed against different backgrounds. Removing the backgrounds and adjusting colors based on lighting are both nontrivial tasks. I think the C/C++ based tools mentioned by tachyon in the first post will be your best bet.