Well if you are looking to build everything from the ground up, you’ll want to start here.
This will let you find all files in a folder, so as long as you know you’re working with //sdcard/images or whatever the path is, you wont need a custom browser. If you would like to build a browser, though, this video will pretty much walk you through it in just a couple minutes.
Once you have found all the files, you can use the scroll view to make a box you can scroll through, though you may not need to if the video works for you.
Lastly, this will teach you how to save and load your files so you can save their image in a place where you can remember it, and obviously you’re going to want to load the images at some point!