Can I retrieve pictures from an iPhone 3G/3GS?

i had a apple iPhone 3 and upgraded to 4. I had pictures on the 3 that were not saved to sims card. The 3 has no sims card in it nowand I connected to my computer and saved pictures to my computer. By accident I then transferred the file back to iPhone and now it say no pictures on the phone and now they are gone from my computer.

If it states that the pics are not on the phone or the computer, consider them gone. It's a good practice to save important pics in multiple locations for retrieval in case you accidentally delete them off of your device.