Black is only transparent if it is in the first palette entry, the purple color is also transparent if put in the second palette entry. The size of a tileset is 320 pixels wide and a multiple of 32 pixels (under 3264 pixels for 1.23 and 13056 pixels for 1.24(TSF)). Just in case someone doesn’t want to use that ‘template’ (which would be most people?) they can just use common sense and follow the instructions JCS help gives.