Yes, I was very surprised when I discovered that some avatars on my board were not resized as they should be. Here are suppositions :
As other boards, IB2 has a setting to restrict avatars size. New uploaded avatars are correctly resized following this setting.
But what happends if you change this settings after that some avatars were uploaded ? I don't think IB2 will resize thousands existing avatars to adapt it with the new setting.
Another explaination : IB2 allows to use avatars hosted by a distant website. In that case, IB2 has no control over avatar's width/height.
I think these 2 examples can explain why they added a width & height parameter in <img> tag.