Help with MB Blog photo please

[BigTrouble]

Please help!

I have tried several times to upload a photo on MN as my profile / blog picture but it says 'photo must be exactly 80 pixels wide' when I attempt it. What on earth does this mean? And how do I get my pic to be 80 pixels wise? I assumed, like with other sites, a photo could just be auto-cropped or whatever but I admit I am challenged technically, as demonstrated here!

Thank you :)