My SVG avatar appears to be causing a 500

```
$ curl -I "https://chat.indieweb.org/img.php?url=https%3A%2F%2Fiambismark.net%2Favatar.svg&sig=a5a2e6906c3379549bf588b963d8365b8f4088ec24084d0bb34cb8b338bb5042"
HTTP/2 500
server: nginx/1.14.0
date: Thu, 26 Jul 2018 01:37:03 GMT
content-type: text/html; charset=UTF-8
x-powered-by: PHP/5.6.36-1+ubuntu16.04.1+deb.sury.org+1
```

Start by reproducing the request with curl against the img.php entry point and inspect its error output or logs for the SVG URL. Trace how img.php fetches and processes SVG avatars, then verify that the reported avatar request returns a successful response instead of HTTP 500.

Written by the indexing model from the issue text.