@soulthoughts I'm sorry for the lack of informations I gave. I'm on iPhone 1.1.1 and I've changed the files Default_CARRIER_ATT.png and FSO_CARRIER_ATT.png using iPhoneBrowser 1.4.0.0. But altough the files in iPhoneBrowser showed changed, the carrier logo still appear as RO Vodafone. MIM as I said, worked. But only text, no png.
Now, I've downloaded the springboard in the warrior888's link, made a backup of my own, copied in springboard.app and as warrior888 stated it forced to show the AT&T logo. Then simply made the changes on png and voila... it works

I don't know how trusted is the blog in warrior888's link, so can someone give me more details?
Thank you very very much both of you guys for helping me.
Now I'll try to make a carrier logo saying iCata (or iCatalin, I don't know witch one shows more properly) with the apple logo at the beginning, using apple font (exactly as iPhono logo on page 4) but if somebody wanna do it please, because I'm an amateur and I don't know how good it would come out.
Edit: I don't want to be a bit pushy but here's a challenge for a Photoshop enthusiast: I wanna modify the signal bars, so when there is no signal (Default_0_Bars.png) the png will be red, 1 bar red, 2 bars a pale red, 3 bars a pale blue, 4 bars blue, 5 bars green. I don't even know if I'm choosing the correct colors that will show good, so if you wanna rectify me, please do so. When I've chosen this set of colors I imagined a stronger and stronger signal...
Once again please excuses my bad english/grammar or stupidity in my request. Thank you very much.