STI pictures tend to be OMFG photos from people with severe - and very obvious - symptoms.
What you're looking for generally are things like warts, sores and pus. If you squeeze their penis, do they go ow or does anything that's not obviously precum come out? They probably have something if so. Just learn what Fordyce spots look like, or you'll be turning away lots of healthy men.
This sort of check cannot be completely effective because lots of STIs don't have any obvious symptoms. If someone has HIV, you're only likely to know it if they tell you or they've gone untreated for so long that their immune system is badly compromised and they're getting other infections.
Reducing the risks is fairly simple, especially if you know your own attitude to risk. 'Obviously' you want to use condoms for any penetration. Beyond that, it's much more down to personal attitude. If catching anything is out of the question, so is doing OWO, because there's a risk there. If you take the attitude that you're going to have a more or less discreet look and squeeze before doing OWO, it might be an acceptable risk for you.
You can also work on a 'this hand touches them, that hand doesn't' system, so that you only touch yourself with the hand that hasn't touched them.