For anyone having the same problem, here is the solution.
Solution
I am using the File System API from the react-native-fetch-blob library. This is because I tought it was way better documented and easier to understand than the 'react-native-fs' library.
I request an image from the server, receive a base64 and I then save it to the Pictures directory in the android fs.
I save the image like this:  
var RNFetchBlob = require('react-native-fetch-blob').default;
const PictureDir = RNFetchBlob.fs.dirs.PictureDir;
getImageAttachment: function(uri_attachment, filename_attachment, mimetype_attachment) {
   return new Promise((RESOLVE, REJECT) => {
   // Fetch attachment
   RNFetchBlob.fetch('GET', config.apiRoot+'/app/'+uri_attachment)
   .then((response) => {
     let base64Str = response.data;
     let imageLocation = PictureDir+'/'+filename_attachment;
     //Save image
     fs.writeFile(imageLocation, base64Str, 'base64');
     console.log("FILE CREATED!!")
     RNFetchBlob.fs.scanFile([ { path : imageLocation, mime : mimetype_attachment } ])
     .then(() => {
       console.log("scan file success")
     })
     .catch((err) => {
       console.log("scan file error")
     })
    }).catch((error) => {
    // error handling
    console.log("Error:", error)
  });
},
The following code that is in the above method refreshes the Gallery otherwise the images would not display untill the phone is turned off and back on again.
RNFetchBlob.fs.scanFile([ { path : imageLocation, mime : mimetype_attachment } ])
.then(() => {
  console.log("scan file success")
})
.catch((err) => {
  console.log("scan file error")
})
Enjoy!