a quick way to resize it is e mail the picture to yourself. I use gmail and it automatically compresses the image. once you get the email you should be able to upload the attached image without any problems.Hope that helps.
You can also print the screen, immediately saved a new and smaller size foto. It's pretty ease on iPhone to. Just click the HOME and POWER button simultaneously =)
You can upload it to a free hosting site like Imgur or tinypic, resize it and then post it to the forum. It must be easy or I'd have never figured it out!
The print screen part - it can be useful to copy text messages, etc. I just posted the local weather forecast on the "What's your weather like?" thread using this.