LadyCFII posted the link, and at the bottom, it says:
"To capture a screen shot on an iOS device (e.g. iPhone, iPad, iPod touch)
Simultaneously press the Home and Power buttons. If your sound isn't muted, you will hear a camera sound and the screen will flash. You can find the captured image in your Photos app on the device, where you can email it to yourself or use another application to share it."
Once you are ready to post it on the forums, you can upload it via the File Manager, as the middle of that same post says.
"If you want to attach a file to your post, you must use the Reply or New Topic function, since the Quick Reply function will not provide access to the file manager. To attach a file to your post, click on File Manager in the Post Options section of the Posting Form, which will open the File Manager Screen. The file(s) you wish to attach can be added using the input field and the buttons. A simple way to specify the file name and location is to use the Choose File button and navigate to the desired file on your computer (in iOS select "Choose Existing"). Once the file name is displayed in the input field, click on the Add button to add that file to the File Manager. The file will be listed at the top of the File Manager page. To add more files, repeat the process for each additional file. As you add files, they will also be displayed at the top of the file Manager page. When you are finished attaching files, click the Done Adding Files button. You will be returned to your post, and the number of files that you added will be displayed next to the File Manager link. After you click Submit, the files will be listed as individual attachments at the bottom of your post. To display an attached picture in your post, you can click on the attachment link, copy the url from your browser, go back and edit your post and use the "Enter an image" button (3rd from the left) to paste the link to your picture into. When you click Change Post, the picture will be displayed in your post. "
That looks like everything you would need to post an iOS screenshot to the forums.