I am no expert with technology but regularly folk seem to have trouble uploading photos.
I was taught the following method via another blog site. If you want to post a photo from Flickr photo stream you just open the individual photo on the photostream page.
On the bottom right hand of the page you will see a row of icons - click on the forward arrow - on the new page highlight the far right option BBCode. You are then left with a window showing the URL link with a second box underneath where you select the picture size from a drop down menu. Copy & paste the whole URL highlighted link into your reply. If you select preview you will be able to view the photo rather than the URL link.
When you post the reply the photos appear instead of the URL links. No size reduction of the picture is needed. You need the photos to be public on Flickr.
Hope this helps folks - I can only do this on my Laptop or PC - not yet managed it on my phone.