First you need to cut the photo into the correct size. You should have a CD rom come with your 6100 which also can change the photo into correct size. But I usually use other software like ACDsee to change the photo into the correct size.
Then you need to upload the photo to the 6100. You can buy a connection cable which is expensive. Or you can transfer it by infra red. If you use notebook, you should have infra red already. If you use desktop, you may need to buy a USB infra red.
You can send photo, ringtones, etc to your 6100 with the infra red, so it is worth to buy one.
If you want to know more, let me know. I have forget the details because I seldomly use it and need to check the details in my office. I install everything in my office computer.