Repository: yt-dlp
Issue: Support ImageMagick for thumbnail post processing
Labels: enhancement,triage,core:post-processor

### DO NOT REMOVE OR SKIP THE ISSUE TEMPLATE

- [X] I understand that I will be **blocked** if I *intentionally* remove or skip any mandatory\* field

### Checklist

- [X] I'm requesting a feature unrelated to a specific site
- [X] I've looked through the [README](https://github.com/yt-dlp/yt-dlp#readme)
- [X] I've verified that I have **updated yt-dlp to nightly or master** ([update instructions](https://github.com/yt-dlp/yt-dlp#update-channels))
- [X] I've searched [known issues](https://github.com/yt-dlp/yt-dlp/issues/3766) and the [bugtracker](https://github.com/yt-dlp/yt-dlp/issues?q=) for similar issues **including closed ones**. DO NOT post duplicates
- [X] I've read the [guidelines for opening an issue](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#opening-an-issue)

### Provide a description that is worded well enough to be understood

Kinda niche issue, thumbnails generated using --convert-thumbnail jpg do not play nicely with rockbox and display in black and white. 

Im wondering if its possible to have ImageMagick support to handle the thumbnail post processing before embedding, simply running convert on the thumbnail from jpg to jpg makes it work

ImageMagick could also be used to handle cropping and other stuff like is being done with ffmpeg currently like in this thread: https://github.com/yt-dlp/yt-dlp/issues/429

### Provide verbose output that clearly demonstrates the problem

- [X] Run **your** yt-dlp command with **-vU** flag added (`yt-dlp -vU <your command line>`)
- [ ] If using API, add `'verbose': True` to `YoutubeDL` params instead
- [X] Copy the WHOLE output (starting with `[debug] Command-line config`) and insert it below

### Complete Verbose Output
```
yt-dlp https://www.youtube.com/watch?v=YHDfpBwxeVU --convert-thumbnail jpg --embed-thumbnail -f bestaudio -x --audio-format mp3 --audio-quality 320k --write-thumbnail
[youtube] Extracting URL: https://www.youtube.com/watch?v=YHDfpBwxeVU
[youtube] YHDfpBwxeVU: Downloading webpage
[youtube] YHDfpBwxeVU: Downloading ios player API JSON
[youtube] YHDfpBwxeVU: Downloading android player API JSON
[youtube] YHDfpBwxeVU: Downloading player 410a4f15
WARNING: [youtube] YHDfpBwxeVU: nsig extraction failed: You may experience throttling for some formats
         n = kqh9ZIBLVf5XJK1kn ; player = https://www.youtube.com/s/player/410a4f15/player_ias.vflset/en_US/base.js
WARNING: [youtube] YHDfpBwxeVU: nsig extraction failed: You may experience throttling for some formats
         n = RC0lxQxGFQVnFZIw6 ; player = https://www.youtube.com/s/player/410a4f15/player_ias.vflset/en_US/base.js
[youtube] YHDfpBwxeVU: Downloading m3u8 information
[info] YHDfpBwxeVU: Downloading 1 format(s): 140
Deleting existing file 10 Things I hate about you - Nightcore Version [YHDfpBwxeVU].webp
[info] Downloading video thumbnail 41 ...
[info] Writing video thumbnail 41 to: 10 Things I hate about you - Nightcore Version [YHDfpBwxeVU].webp
[ThumbnailsConvertor] Converting thumbnail "10 Things I hate about you - Nightcore Version [YHDfpBwxeVU].webp" to jpg
Deleting original file 10 Things I hate about you - Nightcore Version [YHDfpBwxeVU].webp (pass -k to keep)
[download] 10 Things I hate about you - Nightcore Version [YHDfpBwxeVU].mp3 has already been downloaded
[ExtractAudio] Not converting audio 10 Things I hate about you - Nightcore Version [YHDfpBwxeVU].mp3; file is already in target format mp3
[EmbedThumbnail] ffmpeg: Adding thumbnail to "10 Things I hate about you - Nightcore Version [YHDfpBwxeVU].mp3"
```