While I don't include images, videos, and audio directly into the tmedia repo, here are some good sources that I use to make sure many different file formats and sizes work well.
Images:
The USC-SIPI Image Database Png Suite google imagetestsuite pygif testsuite metadata-extractor-images
Audio:
You can download various mixtapes, audio files, and whatnot from Web Archive, which give an easy way to listen to music and test at the same time
Video:
On AVPackets, sometimes the time_base is not set, so don't rely on finding the time base for a stream through a packet
April 8: 2023, Found a bug where if the audio thread is started before the video thread, the function MediaPlayer::get_desync_time will fail.
October 3rd, 2023: av_frame_get_buffer has an alignment of 1, although ffmpeg recommends 0. 0 doesn't work on my machine for proper resizing, and other parts of ffmpeg also sometimes use an align of 1, so I'ma just leave it for now.