Hello,
It isn't difficult. Many applications can do that, and there are even some free ones that will do so. I believe Irfanview is one, for example.
Also, when the contest is over the winning designer will send you the eps file along with a 1000x800 jpeg image.
You can certainly ask the winning designer to send some different sized jpegs to you.
I can only speak for myself in saying it's no trouble and I've done that in 2 of my wins.
Take care,
Tim