I left this issue for impossible, but NOW it´s totally important for my client.
btn_add_to_cart.gif is in the folder "helloglam_en" with the correct extension. The "helloglam" folder is in: skin/frontend/default/
Also in configuration/system/general/design/ in english version created for my site, "helloglam_en" is defined in "Skin (Images / CSS)" path.
Any Idea? Text Translation is currently working, and i don´t know where is the problem. I have read thousands of tutorials about translations and i have made everithing possible and correct.