you can try .png format and you might also consider svg which would be sharp but I do not know if it is supported for the iPad.
Svg is a vector format designed for the web.I would suggest flash which would be sharp but it is not supported by Apple on the iPad. But swf would probably do the trick.
Make sure the image is not being resized by the HTML code for the page. If you export an image that is 400 pixels by 600 pixels then that is the size the code should be using. If an image is scaled by a web browser because the image size and the HTML code don’t agree then you will not get the best result.
I would suggest flash which would be sharp but it is not supported by Apple on the iPad. But swf would probably do the trick.
SWF is one of the file formats for flash.
I have finally figure out how to save files in smaller size with out loose much quality but now I am encounter with the another issue.
Files' (JPEG) typefaces are sharp and legible look perfect on iPad, when the same file (JPEG) imported to iPhone, the typeface was not able to read.
iPad is much bigger than iPhone. Images work perfect for iPad in theory it shouldn't have lack pixel issue.
It really beats me. I don't know how to figure this out. If you have any idea, please let me know your insight.