With no changes at all still there are posts that are published in facebook with images and others don't.
Since you use WordPress SEO and Shareaolic to managed Open Graph meta tags on your site, I'm afraid I won't be able to help here.
Trying with facebook tools/debug don't show erros but still trying again shows error 500.
Then could it be posible that error is when sending post to facebook the returns a 500 error code and then only is visible the link to post?
Is there any way to avoid or delay publishing if error 500 is returned?
Could you give me more details about this? if you get 500 errors when trying to access your site, that might explain the problem. If Facebook can't reach your site when looking for information to build a post preview, then it won't add any image to that preview.
You might want to look at your sites' error logs in order to find out more about the 500 errors. If you do not have access to these logs, your hosting provider should be able to help.