Dropbox requires adding ?raw=1 (or dl=1) for direct binary file access.

Replace:

https://www.dropbox.com/s/nx8ufy3jxc9urgv/ic_reward_3rb.png

With:

https://www.dropbox.com/s/nx8ufy3jxc9urgv/ic_reward_3rb.png?raw=1

OR

https://www.dropbox.com/s/nx8ufy3jxc9urgv/ic_reward_3rb.png?dl=1