Welcome to the Spine forum! 🙂
Special image effects on skeletons can be achieved by using a special shader with your skeleton. You can either create a copy of an existing provided Spine shader and modify it according to your needs, create a new one from scratch, or use an already working thirdparty shader.
For example, this forum thread is describing a custom-built water effect shader used with a Spine skeleton:
Shader Graph to submerge character in water? (Example)
When using thirdparty shaders, note that some settings at your
SkeletonAnimation component need to be changed. You need to disable
Advanced - PMA Vertex Color, use straight alpha export, enable generation of normals and tangents see this documentation section for details:
spine-unity Runtime Documentation: Writing your own shaders