For starters, the following should suffice:
1. There is a
DroidFu library that you can use: it has a custom
ImageView widget which automatically loads the image in bakground.
2. Apart from that you can take a look at Mark Murphy's
ThumbnailAdapter class:
cwac-thumbnail
3. See
this tutorial. It uses a queue based implementation for executing tasks.
4. I started with something like
this but then my code evolved to handle a lot of potential issues like the ones discussed
here and
here. You may also want to read
this answer I wrote here
All the best!