There are many imaging tools that can do the job, I just copied your image into IrfanView (freeware - non-commercial use) and resized it and saved the smaller file. I use IrfanView because it is good at file conversion, opening one file format and converting it to another format and it doesn't loose much detail.
IrfanView is more of an image viewer and file converter but it has some basic editing functions
http://www.irfanview.com/