I've noticed Mp3tag leaves padding inside tags, understandably for the purpose of faster editing later on, as the whole file won't have to be rewritten. However, it seems to be quite impossible to get rid of this null byte padded space. Even when the whole file is rewritten (i.e. when removing cover art), the padding remains there.
It would be lovely if there were either a setting or a button to not have this padding. Personally, the extra processing time is negligible.
Secondly, I noticed a difference in saving cover art between ID3v2.4 UTF8 and v2.3 UTF16. I have no idea what the reason behind this is, nor if it's intended, but ID3v2.4 UTF8 actually changes the image data. Specifically, many null bytes inside the image are duplicated. Surprisingly, this does not seem to alter the image, and extracting the image returns the one without those duplicated bytes.
I was using a 219KB jpeg, which had about 2KB worth of duplicated null bytes.