Hi Mark,
I am sorry for the confusion. Look like you did submit the twitter url but somehow it was hidden in the input form ;(
Anyway, regarding your issue with twitter autoposting not showing the image in twitter summary preview, it was caused by two issues.
1. First issue was caused by EasyBlog not picking up the image correctly. I've fixed this and applied the fix on your site and now in your blog post, the twitter:image tag is showing the correct image URL. For your information, the twitter autoposting require the Twitter card to be enabled from your site so that twitter crawler can get the image path from twitter:image tag.
2. Twitter crawler was not able to 'crawl' the image link due to your server robots.txt prevent any robots crawler to access images folder. I've modified your Joomla site robots.txt and commended out the line 19 so that twitter crawler can crawl your image link. Please see screencast.com/t/qeloXS8Ynq
Now, in your twitter, the image is showing correctly. Please see
http://screencast.com/t/m2xeKOfcJL
Hope this help and have a nice day!