You can dowload a copy of IrfanView from:
http://www.irfanview.net/ (It's free. And it doesn't contain any tracking stuff or other junk that you don't want.)
Open your file.
Then click on image / resize-resample.
Make sure 'preserve aspect ratio' is checked and then set the new size to 80 <<-- you should set the BIGGER number to 80 (the smaller number will automatically resize). (For example, if your picture is 500 x 600, you would change the '600' to 80; the 500 would automatically change to '66'.)
Then save the file with a NEW name. I usually put "th' at the beginning or end of the new file name so that I know it's a thumbnail image.
There ya' go!! You can use that new file for either an avatar or a siggy pic.
Good luck!!
Hugs.
Barb