This is only possible if the images are in .psd format.

well yes 2 ways:-
1) the previous text is invisible/size 0px/doesn't exist at all.

2) the hard way: if some addition is to be done, should be easy.
if very little modification needs to be done, then using brain might help, like when converting 3 to 8
if huge mod needs to be done (complete change of text) and the stratergy of little mod does not works, then erase the text and try recreate the original image background, after you have typed the text.

if its an animated gif, things should be more easy, but can get more difficult too. some grey matter will be required.

in want text editing software in ubuntu

Open the image in photoshop and change to cmyk or rgb mode if it is a index. Add a new layer and add you text and again save the image as gif from the option save as web.

I personally use Fireworks program to help.

It's impossible to edit i think . SO i think you need to have a PSD file