If your a Pc, you can resize in Paint
. Right click on your pic, scroll down to OPEN IN PAINT
a new box will open with your picture in it, scroll over to IMAGE scroll down to resize/skew in the RESIZE change the values of both horizontal and vertical, change to 50% then your pic will resize, You need to scroll over to FILE and scroll down to SAVE AS, another box will open up asking you to name your new size picture. Name it with the word web, forum, etc in the name that way when you need it again you know which picture has been resized for internet/fourm uploads. But you have to SAVE AS or you will lose your orignial. Good Luck!
Pic's help for buyers to see what they are buying, if you have a Pc the above info can get the pic to the right size to post. Good Luck with your sale, sounds like a nice trailer.