I use your MP3tag for years now and still love this great program ! It would be really great if you guys can add for OPUS (the best "Lossy" Audio Compression Format today), the Cover Art option! We can do this with a horrible, boring and long command line (see the command line below) but apparently and sadly, there is no GUI version on the Internet for this.
opusenc - --picture command line:
--picture filename|specification (Attach album art for the track.)
Either a filename for the artwork or a more complete specification form can be used. The picture is added to a METADATA_BLOCK_PICTURE comment field similar to what is used in the specification is a string whose parts are separated by | (pipe) characters. Some parts may be left empty to invoke default values. Passing a plain filename is just shorthand for the "||||filename" specification. The format of specification is [type]|[mime-type]|[description]|[widthxheightxdepth[/colors]]|filename type is an optional number describing the nature of the picture. Defined values are from one of:
1: 32x32 pixel 'file icon' (PNG only)
2: Other file icon
3: Cover (front)
4: Cover (back)
5: Leaflet page
6: Media (e.g., label side of a CD)
7: Lead artist/lead performer/soloist
12: Lyricist/text writer
13: Recording location
14: During recording
15: During performance
16: Movie/video screen capture
17: A bright colored fish
19: Band/artist logotype
20: Publisher/studio logotype
The default is 3 (front cover). More than one --picture option can be specified to attach multiple pictures. There may only be one picture each of type 1 and 2 in a file.
mime-type is optional. If left blank, it will be detected from the file. For best compatibility with players, use pictures with a mime-type of image/jpeg or image/png. The mime-type can also be "-->" to mean that filename is actually a URL to an image, though this use is discouraged. The file at the URL will not be fetched. The URL itself is stored in the comment field.
description is optional. The default is an empty string.
The next part specifies the resolution and color information. If the mime-type is image/jpeg, image/png, or image/gif, you can usually leave this empty and they can be detected from the file. Otherwise, you must specify the width in pixels, height in pixels, and color depth in bits-per-pixel. If the image has indexed colors you should also specify the number of colors used. If possible, these are checked against the file for accuracy.
filename is the path to the picture file to be imported, or the URL if the mime-type is -->.
OPUS Official Website: opus-codec.org
OPUS codec comparison: Audio Codecs Comparison
Encoder/Decoder for Windows and MAC (Command Line): opus-codec.org - Downloads
Or here - (Windows Only): rarewares.org - Opus Encoder/Decoder Tools Downloads
(Works with Foobar2000, Speek's Encoder Frontend, etc.)