Try using Irfanview. It's a superb (free) image viewing and manipulation program, which deserves a place on everyone's PC:
http://www.irfanview.com/
Open your image in Irfanview. Then go to Image > Resize/Resample.
Check that the bottom right-hand corner of the panel shows the usual default setting of 'Resample' (rather than 'Resize'), using the Lanczos filter. (Resampling will give a better quality image than resizing). Choose your size and click 'OK'. Then go to File > Save As. (NB: DO NOT USE FILE > SAVE. If you do so, you'll over-write your original file). Choose a suitable name and location for your image.
To reduce the file size further, without reducing the visible image quality, also try reducing the DPI setting (e.g. from 72 to 36) at the same time as you choose new dimensions for your image.
Chris
(PS: My post assumes that you've not got access to Photoshop. If you have, use the File > Save for Web function).