Thanks for the quick replies.
I have been testing the solutions:
I can use g.region to create different regions (with specific grid size) so they can match the resolution of the tifs i am importing (this will be useful because the tifs have different resolutions). After that r.region -c will recompute the image to the extents of the current region. This seems to be what i needed 
Again, thank you
Ricardo
On Tue, 20 Mar 2007 13:19:23 -0700, âAmaginary Studiosâ
amagine@telus.net said:
Hmmm,
I believe you will need to specify the correct âresolutionâ in your
âg.regionâ settings which will ⌠match the resolution of your .tiff
files.tfw files are simply Text files so I suppose you could create your
own within a text editor.for example a .twf file with the following
1 nresolution
0 x-pixel position reference point
0 y-pixel position reference point
-1 sresolution
0 x-postion reference
0 y-position reference
says, that each pixel in the tiff image is equal to 1x1 (in your
case probably meters). and the Top left corner is referenced at 0m x
0m.So, it is a matter of matching your image resolution size to the
correct resolution region settings in GRASS.I hope this helps a bitâŚ
Mars