Thank you so much,
That worked perfectly. I used 640×392 pixels and immediately the image got where it should be.

Mistery solved, thanks to you. The images that worked wre the ones that had the same aspect ratio, and the ones that didn't failed to fit that aspect ratio.

Again, thank you so much,
Nuno Ribeiro.

When it comes to featured images they must be the exact dimensions specified by the theme. You will find those in the articles that I linked to above.
You can also consult these troubleshooting images tips.

blog url: http://conversasnotanque.wordpress.com/
blog template: "Structure" by Organic Themes

I have this problem for months. This template has what is called a "featured image". You chose it when you are writing a new topic, on the right side, clicking a link called "set feature image". That image shows on the top of the blog, along with the title of the post it is associated with. It worked without any problem. But, some months ago, randomly, some images started not showing up. They still show up in the post column, in the post preview they are associated with. But, if you click to see the post, they do not show up in the full view post.

I have tried many different things.

1 - I imagined it was a Copyrights protection from WordPress. Almost all of the images we ( we are a non-profit) use are photos we take ourselves. This one, a orange, abstract-ish photo, for example, is taken from a DeviantArt profile of an artist that explicitly allowed it to be downloaded. This has happened, many times, with photos that we have taken ourselves with different digital cameras and even, one time, with a cell phone.

2 - The problem could be the size/resolution. The crop is different, on the top, where the feature article title is, where the feature photo appears. So I experimented with different sizes/resolutions. And nothing worked. When WordPress (or this template) does not like one image, nothing seems to work. I just have to give up and try another, and another, and another, until one works. Latelly, it got so bad, that the last post, I just gave up, and we had our last post with a blank space, where the feature image should be.

3 - I also tried with diferent formats. Not too many, only .png and .jpg, but the same logic, if WordPress does not like one image, I cannot trick it just by saving it with a different format.

This is becoming a big trouble. I would be forever gratefull if anyone could help me with this.

Thank you for your time,
Nuno Ribeiro