very easy tutorial for windows users
open the picture you want an avatar of in paint
select the dotted square box left of the screen and use it to crop out the part you want your avatar to be...
make sure that it's the same lenght and width! like, 100 x 100, 555x555, the size doesn't matter, they just need to be the same!
you'll see it's wayyy too big for your avi
save the file (use bmp for now, it doesn't make you lose quality!)
download the free program Irfanview (use google to find it
)
open the saved avatar with Irfanview
go to Image -> Resize/resample
make SURE the box 'preserve aspect ratio is ticked'
type 72x72 pixels
File --> Save as, select JPEG instead of bmp.... and save it ^^