Convert canva photo to piskel - reduce colours/resolution

How do I…

Convert a basic image (it is a photo of a mahjong tile) that I uploaded from canva to piskel as a png so that it doesn’t have performance issues

What is the expected result

I want to use an image (which is basic) that I took a photo of as a piskel, but when I edit the image to crop/resize it, I get thrown ‘performance issue’ warnings. I think the photo contains too many colours but i don’t know how to limit them.

What is the actual result

“The current sprite may exceed the recommendations for Piskel” : Try adjusting your sprite settings. I believe the photo has too many shades of grey, which I am happy to get rid of, but this is my first rodeo with Gdevlop and I only have previous (basic) experience with Python.

TIA

NikS