Depends on how you do it.
I keep a folder on my desktop to sync -- family photos, photos of my dogs, wallpapers, etc. You can tell iTunes to sync images from a folder in the photos tab on iTunes. I call the folder "iPhone" and then have subfolders inside with "family" "dogs" "wallpapers" "Nationals" etc that I can pull up in photos on my phone.
If you're on a Mac I'm sure you can pick something from iPhoto but I prefer the folder method.
If it's just one photo you need can you email it to yourself? You can save it from there for whatever your need is.
If iTunes is not able to see your phone at all, try a different cable or unplug it and plug again to see if it recognizes.