I am currently using Stack Exchange using Safari on my iPhone 4s. How can I upload a diagram to this site from the camera roll?
Currently, the mobile theme of Stack Exchange sites does not support image uploads, it got only the plain textbox editor without any buttons whatsoever.
There is a pending feature request asking to add it, but it might take years, if at all, to be done.
Until then, you can switch to "full site" mode on the mobile device:
Now you can just tap the image upload icon in the editor and upload same way you do it in a desktop computer.
Another option is upload the picture to a different site, e.g. imgur.com and using the raw markup:
Or if you feel more comforable, raw HTML tag:
<img src="URL" />