Unfortunately, there is no endpoint to upload category image, I worked on this case before, and i would like to share my solution
first of all, you need to build a custom endpoint to upload the image to this path (send the data in base64 then save it in this path)
/{magento_root}/pub/media/catalog/category
lets say your image name is your_image.jpg
after you save this image in this path, you can send the data using the standard magento endpoint
/all/V1/categories
{
"category": {
"parent_id": "2",
"name": "cat_name",
"is_active": true,
"include_in_menu": true,
"custom_attributes": [
{
"attribute_code": "image",
"value": "your_image.jpg"
}
]
}
}