upload your new icon anywhere in the web server e.g. webroot/images/heart.png and then point the Image replacement URL entry to it. In this example, /images/heart.png
As was already mentioned, you just change the image replacement URL to be the path to the desired image on your server, but if the reaction is already a sprite then you need to disable sprite mode:
You should be able to download all of the emojis for free from JoyPixels' website:
You just need the 32px version for the image replacement URL but for best results also download the 64px version and use that for the image replacement URL x2 as this will be served on devices with a higher pixel density instead.