I'm using picasso library to load images for my app. But I don't how to implement my own disk (sdcard) caching with picasso library.
Picasso uses the HTTP client for disk caching and if one is already configured it will use that instead of installing its own.
For the built-in UrlConnection the docs for installing a cache are here: https://developer.android.com/reference/android/net/http/HttpResponseCache.html
If you are using OkHttp then you just call setCache: http://square.github.io/okhttp/2.x/okhttp/com/squareup/okhttp/OkHttpClient.html#setCache-com.squareup.okhttp.Cache-