Just get imageMagick, a command line image editor you can get from MacPorts. I made a bash script to automatically resize and tile at 100%, 50% and 25% resolutions.

tile.sh:
#!/bin/bash
file=$1
function tile() {
convert $file -scale ${s}%x -crop 256x256 \
-set filename:tile "%[fx:page.x/256]_%[fx:page.y/256]" \
+repage +adjoin "${file%.*}_${s}_%[filename:tile].${file#*.}"
}
s=100
tile
s=50
tile
s=25
tile

If you run: tile.sh bigimage.jpg it’ll dump out 256×256 sized tiles called bigimage_100_0_0.jpg, bigimage_100_0_1.jpg etc for each tile at 100% scale, as well as bigimage_50_0_0.jpg et al at 50% scale etc.