I do screenshots from my iPhone quite often.

Get whatever you want to show us on your screen, then press your home button and power button quickly at the same time until your screen flashes, this will create a screenshot in your photo folder.

Then, if you scroll down to the bottom of the forum page on the iPhone you can click an option to view the forums in 'desktop' mode, so in effect you will be able to do what we can on the PC.

Then, go to write a new message, and click the little square with the tree on to insert an image.
Swipe to zoom out so the 'insert image' box is in view.
Click the 'from computer tab' and then click the choose file button.
Select 'Choose existing' and find the image on your phone.
Once youve selected the image you want, click the text underneath the 'Choose file' button that says 'Upload File(s)'

The image should now be as an attachment in your post