for websites don't use png24. the large file size of png24 make it not suitable for web pages. they are as large as 24bit or 32bit bitmaps because png24 is lossless or close to it.
it is actually more powerful than bitmap because it captures transparency very precisely. it also has layers, an extremely powerful feature.
PNG-24 is suitable for compressing continuous-tone images; however, it produces much larger files than JPEG format. (See About file formats.)
The advantage of using PNG-24 is that it can preserve up to 256 levels of transparency in an image. To save an image with multilevel transparency, select Transparency. For more information on the Interlaced, Transparency, and Matte options