Need logic behind these apps - Google Image search using keywords

I am planning to build a functionality where searching with a keyword will give images.
https://play.google.com/store/apps/details?id=com.mwxapps.imagesearch
https://play.google.com/store/apps/details?id=sansunsen3.imagesearcher

Need the logic behind these apps. If we can fetch the images without the api, it will be good.