Resizing from a computer is simple if you have Paint...most computers do.What I did was make the post from the computer, emailed pics to my phone and put them in using Tapatalk. I'm sure there are other options but is what worked for me in a pinch
Right click on the image,
Click "open with"
It will open your image with all sorts of option including "resize". You should be able to resize by pixel or percentage.
BE SURE TO MAKE A COPY FIRST!! OR YOU WILL LOSE ORIGINAL RESOLUTION!!!
You can even paint some extra inches on your antlers before entering your harvest in the contest.