this is actually something I've been wanting addressed for a long time. ATBBS does it, so the code should be simple enough to adapt by someone who is familiar with kx.

although, I've never experienced errors related to images being duplicated on another board, only same board pics. If we could get inter-board links to work like they do for inner-board (like ylilauta) that might be an acceptable substitute for image recycling like ATBBS does.

aside from that, you can use jpegcrop to alter .jpg image files without affecting the quality of the image itself. in the settings check the 'huffman optimized' button and 'progressive coding' box.

it changes the method in which the compressed data is stored, to make it more efficient. this has the end result of not only making the file size decrease, but the file will have a different md5 hash which won't trigger the duplicate message when posted. (and since only 1% or less of jpeg images on the internet are optimized, you're virtually guaranteed the file will be altered enough to upload.)

for png images, you can use optipng, pngout, or deflopt http://www.mediafire.com/?gnd1tdv96h5io2z
they will only alter the file if it can be made smaller, and all are visually lossless. just be careful if you have any apng files, pngout and deflopt will remove the animation. (apng thumbnails won't be animated when posted, and the animation only works in some browsers.)

there's utilities for optimizing gif images out there, but all I've found are either trial versions/pay to use, or not actually lossless. best you can do is google one with different dimensions, or open it in gimp to manually alter it by smearing a pixel or something.