Well, unfortunately, without really seeing first hand what is going on, it's doubtful that anyone is going to be able to help you other than to tell you it's style related most likely.
And if you are talking about default avatars, it will be under the style directory for that style. If you are modifying it to customize it for your site, hopefully you are doing it to a child style and not the base one. Otherwise, when you upgrade the base one, all your mods will be lost.
Look for the avatars in the /styles directory. You can find the default path for the images for the style in the ACP under ACP -> Appearance -> Style Properties -> General -> Path to Images.