To choose any custom image to use as a Reaction navigate to Reactions » Reactions Manager and click any of the existing Reaction image, a drawer with a collection of images will open.
Click on the button over the collection to open the Media Library and Uploader and use any image you want.
Bitmap images such as JPEG, PNG and GIF cannot be colorized yet, using the color picker widget provided.
Vector images should be monochromatic to work properly with the color picker widget.
DaReactions does not enable SVG upload through WordPress’ Media Uploader, you should use a third party plugin if you want to upload SVG from your hard disk.