I dunno why it says KIB (I gotta fix that...) it means KB or KiloByte...
To shrink my Avatars I use a free application called GIMP... its an all around image editing program and it is free.
First download GIMP at:
http://www.gimp.org/Then install it...
Once installed, open GIMP and then open your image inside GIMP.
If your image is rectangular you can crop it to make it square using the "IMAGE" pull-down menu op the top of the screen and select "Canvas size"... More on this later if you need help with it.
Now resize the image using the "IMAGE" pull-down menu on top of the screen and select "Scale Image". Assuming it is square, make it 90x90... If it is not, make sure the largest dimension is 90. The other dimension will automatically scale.
Now that the image is resized we must save it as a JPG image.
Select File / Save As. Pick a destination directory (I just use the DESKTOP) and give it a name "MyAvatar.jpg" for example. A few windows will appear. If given the option, select "Export" and then "Save". Export only appears if the image is not already a JPG.
At 90x90 or less the image should be <6KB now. If it is not, save it again and select a higher compression on the slider bar when given the option to save.
I hope this help!