There are three major image loading libraries on Android. Their purpose is to manage network requests, caching, and display of images loaded from your web server. This is a key component in many apps and I'll walk your through how to pick one.

Picasso & Glide

I lumped these first two together because they function very similarly internally. Based entirely in the standard Android heap, these represent a straight forward and simple solution to image loading. You access a singleton, with a url and target, then the library does the rest. Its a short and sweet single line with no other required set up.

Picasso

Picasso.with(ctx).load("https://placekitten.com/g/720/480").into(iv)

Glide

Glide.with(ctx).load("https://placekitten.com/g/720/480").into(iv)

As you can see they are nearly identical. The difference is that Glide is configured for better memory performance right out of the box. It decodes images as Bitmap.Config.RGB_565 and resizes them to fit the view they'll occupy. Together these two configurations lead to fewer memory spikes and lower sustained memory usage.

Its worth noting that Picasso can be configured to work near identically, but its not the default. At the time of this writing Picasso 2.5.2 is 18 months old and contains some unfixed edge case bugs you'll need to manually patch. Glide remains in active development and maintains more regular releases.

Fresco

Fresco is a product of Facebook's engineering team and backed by ashmem, making it a significantly different approach to nearly all other open source solutions. It is in heavy development but used on a massive scale. It also displays animated GIFs and progressive JPEGs giving it a leg up if you need support for either.

The biggest down side is the implementation. If you are dealing with a legacy codebase that loads a ton of images or has specialized needs you'll be hard pressed to adopt Fresco in a week, let alone a day. Since its such a different pattern and has its own special rules, you'll want to be 100% sure you like it before making a call one way or the other.

// In your custom Application.java
Fresco.initialize(ctx)

// Load image
draweeView.setImageURI(Uri.parse("https://placekitten.com/g/720/480"))

As you might have noticed, draweeView looks a little odd and it requires an initialization call. With Fresco you cannot use a standard ImageView due to its optimizations around memory usage. One last down side here is that if you have a custom ImageView that applies an effect, Fresco could make it way more complicated and you'll be stuck trying to make it work rather than using something off the self.

Recommendation

If you load a low to moderate amount of images Glide is a great choice with complexity built in if you need it down the road.
If you need GIFs, progressive JPEGs, really care about memory, or push a ton of images through your app go, with Fresco.
If you are totally into Square engineering (and who isn't?) go with Picasso.