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?