There are tons of programs that will let you resize images. You can do a search on
google(remember, google is your friend) and find a bunch. I'd recommend a free one called IrfanView though. You can download it from
HERE.
Once you've downloaded it and installed it, open up the image you want to resize using IrfanView. Then click the
Image menu and select
Resize/Resample. Then type in the dimensions you want where it says
Set new size: and click OK. Then save the image and upload it to DDM.
Hope that helps.