I use a free picture viewer call IRFanView. http://www.irfanview.com/
Open a picture with it and then select Image > Resize.
I keep them around 1024x768 to post here.
I sometimes open my pic up with paint and click on image. From there I click on the drop down that says resiez/skew and will resize from there. Good luck to you! I also sometimes use Microsoft Digital Image Editor.. I hope this helps you..