There is a limit to the size of the image you can upload:
Allowed file types: txt, bmp, jpg, jpeg, gif, pdf, mpg, png, doc, pdf
Restrictions: 4 per post, maximum total size 1600KB, maximum individual size 400KB.
On the iPhone you can select image size before you upload. See bottom of the screen on your phone.
The one I attached here from my iPhone is 4.2 mb in original size, but I uploaded it in medium size to get it under 400kb.