Okay, I've scaled it down to 1080 pixels in height as you suggested which brings it down to 1.5MB. I had to preserve the PNG format in order to maintain transparency.
The PNG is an art piece hand-drawn by a friend, and I wanted to preserve its quality. What is a suitable size range I should try to downscale the image to?
It seems there may be some confusion. This project is an RSS/Atom feed notifier; it is not a notification server. It can be used with any notification server implementing the FreeDesktop Notifications Specification such as Mako, not act as an alternative. Please share your thoughts if you try it out!