Don't panic, we've all been there!
I use photoshop elements to resize my photo's but any decent photo program should be able to let you resize the picture.
In elements I select the photo I want then on the FILE in the tool bar select save for web.
The images are loaded and down the right hand side I just put the longest measurement to 700pixels the other moves with it.
I then increase or decrease the amount of compression(its on a scale just above the resize box) I do this so that the size of file is less than 240kb shown at the bottom of the image.
I then save this in a file and when you reply to a post click on the browse button to locate your file.
Sorry for the very poor description and you may not have elements, in the newbie subforum I think there is a written procedure with screen shots on how to upload the pictures.
Hope it helps a bit still not very good with all this myself.