Embedding cover art into flac files -- Method used by MP3tag

On the slim devices forum it is suggested that there are two different methods for embedding cover art into a flac file, one of which is preferable:

"If you're going to embed cover art, I'd use FLAC's preferred method over the Vorbis comment method. The former is more likely to enjoy widespread support with the passing of time."

Is this true? If so, which method does MP3tag use?

On a related question, does the compression level of a jpg cover image affect how much space the image takes in the flac file? If the image is 300x300 pixels and takes 50KB of space on disc as a jpg image, how much padding should one insert into the file to accommodate the image? (The compression question relates to whether the 50KB matters, or whether all that matters is the image size (300x300).) One persion on the slimserver forum suggested adding 20KB of padding which seems too little.


I cannot answer the first question, but for the second question it seems logical for the padding to be at least as large as the filesize of the image you want to add if you don't want the whole file to be rewritten.
For JPG, there is no relation between image dimensions and file size - it all comes down to how complex the image is. A complete black 1024x768 image compressed with JPG will probably be smaller than a 800x600 image containing pink noise. Therefore, 20 KB padding might be too little if you want to add an image that can take up at least 30 KB for example.

Mp3tag uses "FLAC's preferred method" which was introduced with FLAC 1.1.3