Hello Esteban,

Regarding:

When users place the main picture for this article have different formats. I want it to be automatically adjusted to 900 x 305 in the front. Is this possible?

What do you mean by main picture? Do you mean blog image? If yes, you can configure it here: Joomla > Components > EasyBlog > Themes > (Select theme) > Blog Image. Here is the screenshot: http://screencast.com/t/692jmLZvA . If no, can you explain more and provide us with an appropriate screenshot so that we are able to understand you well and assist you on this issue.

Hope this will help.
Thanks.