Hello Karl,
Thank you VERY much for your answer.
Our problem is not that the images are with low resolution. Since they all came from screenshots, we cannot change their quality.
The problem was that the images were rendered\printed too big, and they get blured. Have a look into the attached PDF.
First image, has a width size defined look like we want to.
The second image, without any width set, "automaticly" print the image to 72 dpi, looking quite ugly.
We have been setting the widths for each single image, and its quite borring, because we have loads of images of diferent sizes.
The idea was to force the images to print at around 135 dpi instead. I thought it was controlled somehow in texi2pdf. But you gave me the right hint by saying it wasn't!
In fact, all I had to do, was to open the image in GIMP, and set the print size to 135 resolution for both X and Y, and as you can see on the third image (in the next page) without any width size, the image "prints" at the desired resolution.
Problem solved!!
In behalf of QGIS Project, thank you again.
Alexandre Neto