Bug: avatars are not displayed in the correct format in the timeline

**Describe the bug**
Hi,

I found out that avatars are not displayed in the correct format in the timeline (.jpg) while the original one are .png or .gif. BTW, they are displayed correctly in profile page.

- Which site: [dev.phanpy.social](https://dev.phanpy.social/)
- Which site version: 2024.06.23.5d6a43e
- Which instance: **It's a GoToSocial server, and I can't reproduce it on a Mastodon server, so maybe it's also related.**

**To Reproduce**
Steps to reproduce the behavior:
1. Go to the timeline
2. Find out that some users' avatars are with black background
3. Use browser dev tool to check avatars' format, they're .jpg instead of .png or .gif

**Expected behavior**
They should be displayed as the original format, or at least without the weird background.

**Screenshots**
Timeline or post avatar:

Profile avatar:

**Desktop (please complete the following information):**
- OS: Linux
- Browser: Firefox
- Version: 107

**Additional context**
N/A

Start by reproducing the issue in the timeline on a GoToSocial server and inspect how avatar images are rendered there, then compare that behavior with the profile page where avatars display correctly. Done means timeline avatars retain their original format or display without the black background, including for the reported Firefox environment.

Written by the indexing model from the issue text.