Besides those, you'd need to install a local HTTP proxy (with TLS support) that allows you to modify headers that you run Opera through.

Proxomitron and Proximodo were the ones that I remember that could do that, but those are pretty old and I haven't used them in years.

Create a folder named "nowebp" and then, create the following two files in it:

manifest.json:

{
  "manifest_version": 3,
  "name": "Set Accept Header",
  "version": "1.0",
  "description": "Sets the Accept request header for all URLs",
  "permissions": [
    "declarativeNetRequest",
    "declarativeNetRequestWithHostAccess"
  ],
  "host_permissions": [
    "<all_urls>"
  ],
  "declarative_net_request": {
    "rule_resources": [
      {
        "id": "ruleset_1",
        "enabled": true,
        "path": "rules.json"
      }
    ]
  }
}

rules.json:

[
  {
    "id": 1,
    "priority": 1,
    "action": {
      "type": "modifyHeaders",
      "requestHeaders": [
        {
          "header": "Accept",
          "operation": "set",
          "value": "text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7"
        }
      ]
    },
    "condition": {
      "urlFilter": "*",
      "resourceTypes": [
        "main_frame",
        "sub_frame",
        "stylesheet",
        "script",
        "image",
        "object",
        "xmlhttprequest",
        "ping",
        "csp_report",
        "media",
        "font",
        "websocket",
        "other"
      ]
    }
  }
]

Then, goto the URL opera://extensions, turn on developer mode, click "Load unpacked" and point it to the nowebp folder. Then, set "allow access to search page results" for the extension if needed.

And, of course, there are extensions at https://chromewebstore.google.com/search/save image as that allow you to right-click an image and save it as a certain type.

Besides those, you'd need to install a local HTTP proxy (with TLS support) that allows you to modify headers that you run Opera through.