If you want to upload a product image using Rest API you need to pass the below parameters in your request:

post to https://{YOUDOMAIN}/rest/V1/products/{SKU}/media with body, JSON:

{
    "entry": {
        "media_type": "image",
        "label": null,
        "position": 5,
        "disabled": false,
        "types": [
            "image",
            "small_image",
            "thumbnail"
        ],
           "file": "test.jpg",
        "content": {
          "base64_encoded_data": "{BASE64_IMAGE}",
          "type": "image/jpeg",
          "name": "test.jpg"
        }
    }
}

And if you want, before sending an image, you can encode it to base64 like this:

<?php
$path = 'path/to/image.png';
$type = pathinfo($path, PATHINFO_EXTENSION);
$data = file_get_contents($path);
$base64 = 'data:image/' . $type . ';base64,' . base64_encode($data);

Note: you need to have a header "Authorization" with "Bearer {Magento 2 TOKEN}". How to get it? Just send POST request with your email and password to https://{YOURDOMAIN}/rest/V1/integration/admin/token?username={ADMIN_NAME}&password={ADMIN_PASSWORD} or generate a token via Magento 2 admin panel.