A few thoughts. Most people who are in prison for lesser offences, certainly in the UK, will be in for a relatively short time and will likely have plenty of support from family and friends. They won't be the ones wanting penpals. The ones wanting penpals and I would expect the men would want female penpals, will be the ones lacking family support. These will, in the majority of cases, be those who have committed the worst of crimes. Ditto people in and out of prison for most of their life.
It's also worth considering their motives too.
I'm not saying that there aren't people in prison who stole nappies for babies, or were forced into gangs and into committing horrible crimes against their will. There are some sad stories. But these people won't be seeking comfort from someone thousands of miles away.
I don't know if you like Louis Theroux? Watch his prison documentaries. It's astonishing how charismatic some of the prisoners are. In the Miami one there's a young man with glasses who Louis and everyone watching felt so sorry for. He looked so out of place in the really horrible and violent prison. It turned out he tried to kill his girlfriend and her friend. He shot them. He then announced he was going to plead insanity. There was nothing insane about him. Despite this, I still felt sorry for him.
The San Quentin episode is incredible too. There's a guy given 521 years plus 11 life sentences who seems really nice. He'd been in and out of prison since he was 11. I didn't find him remotely scary. He'd tortured people and when Louis asked if there were any fatalities, he said no, but some people probably wished they were dead, obviously because the torture was so horrific. But I actually liked him. It sounds completely mad as I have a background in law and Social Sciences, so I know the score with people who offend. But I still liked him.
All I'm saying is be very careful. Do not provide anything that could identify you. If you send photos, it's highly likely it will be shared with other prisoners and I am sorry to be brutally frank, but they will not be commenting on how blue really suits you. Many prisoners have access to the internet, either directly or with the assistance of someone else. If they can find you online, they will.
There will be people forced through one reason or another into crime. But it's naive to see them only as victims. If you have committed a serious act against another human being or animal, it will change you. Prison changes people.
Research this properly. Don't provide any personal info and definately do not send photos. There are some good people in prison, but most aren't. Keep safe.