Unfortunately, not all PNGs are created equal. I've run into this problem while using Inkscape. When I try to export an inkscape .svg as a .png, by using the Inkscape PNG export feature, Slick can't load it. I have to open the .svg in The Gimp and then save as a png there.
You may be running into the same problem, where the CS1 png export is incompatible with what Tiled is expecting to use. You could try opening the CS1 image in TheGimp and saving as a png, then importing that png as a tileset in Tiled.
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