How to maintain text clarity of image while zooming?

I am developing an app for a magazine and simultaneously I am creating images for those magazines using photo shop.

But, when I zoom these images, the text quality is getting loosed. I have referred to other iPad magazines like India Today. In that, even though I zoom to maximum extent, text quality is too good.

how is that possible? My each image size is almost 800 KB. Even then, when I zoom, I loose the clarity of image. How to maintain the quality of text even when I zoom?

Is that something to do with photoshop editing or coding?

Solution

We use the following table for our iPhone/iPad projects when the OS target is 3.2 or above.

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