Zoom in an image with out loosing text resolution

Zoom in an image with out loosing text resolution

I want to implement an application like "The star Epaper" available on
Android and Apple Markets. This app is pretty amazing: it uses an small
images but if u pinch this image, the quality of texts never change. It
means by zoom in or zoom out the quality of image is still same. I went to
files of app and found that there is a jpg image ( i.e. 183 KB) and each
image has another file "22 KB" with no-extension. I used openwith and
found that this file is also an image. Therefore, This app is using both
of these images ( maybe one of the is vector) to replace the good quality
for text file whenever pinch event fired.
What is this technique ? and how to implement it?