Hi all,
After a time searching and doing tests, I could see Snagit and Jig are both interesting softwares, but waste me a lot of time to use. Although, I couldn't find a solution in moodle to do it faster and using just a text editor.
So, what I'm doing is using HTML tags. The <figure> allows me to use the <img> tag with the <figcaption> attribute to set up the legend. With this resource, is possible to see the legend bellow of the image (default visualization of captions) just in case your image is aligned alone in the line. If you choose to use text aside the image, your caption will take the same alignment then the text.
This is an example of <figure> tag use:
<figure style="text-align: center;">
<img src="http://omg.sistemaar.com.br/draftfile.php?file=/3625/user/draft/834728035/projeto_grafico.png" alt="Alt de Figura SistemaAR" height="62" width="800" />
<figcaption>Fig1. - Um Exemplo de Sistemar.</figcaption>
</figure>