max avatar size is 100x100. there may be a file size limit as well but i have never hit it. once submitted they also need to get approved by us as we filter out any offensive ones. if you still have issues when trying to upload a correctly sized image let us know so we can have it checked out. unless we specifically remove priviledges for avatars due to abuse, everyone can have an avatar.. there is no need to earn the feature.
the system information area in your profile needs to be unlocked. i believe you need 10 posts to do this. this is done to prevent spam. this information shows up when you hover over someones avatar. you can try hovering over mine if you want an example.
there is no need to specifically have your system information displayed in your signature. it seems people either do it to show off or make questions regarding their systems go a bit faster. for instance, i've never had information in my signature nor will i ever likely put it there. personal preference. generally as long as any images used are limited to one only and are not over the size limit and content is not advertising related or offensive we do not care about signatures