How to upload photo using Postimage
Once clicking on Add image to post The Postimage screen will appear. Don't worry about resizing your pic just click on 'Choose images'.
You will be given an option to upload photo from 'camera', 'documents' or 'gallery'. Click on option where you have your pic saved. Say you chose 'gallery', your gallery will open. Scroll to you pic and click on it. The image address will appear in the 'Reply box'...like this. Click on 'Preview', check if your post is all good and 'Submit'.
If you wish to upload the original sized pic, when you are 'Previewing' your photo left click on it and it will open in Postimage. Right click on pic and go to 'Copy image address' and copy. Go back to you reply box and clear the 'old' image address completely and replace it with the copied image address in between the Img brackets...then preview again.
These steps apply to my smartphone and PC...hope they do to your's as well. Let me know if you still experience any problems or you wish me to expand a little more