What am I allowed to envision when I mention HaShem in prayers?

You are not allowed to envision any image whatsoever when you mention HaShem in prayers. This is absolutely forbidden - it's avodah zarah, idol worship. HaShem has no body, no arms, no face, no back, nothing physical at all. You cannot visualize an image of any kind - not a hand, not a face, not even an old man with a beard. Many people make this mistake, trying to humanize HaShem, but if you're visualizing any physical form, you're committing a serious sin. Some people try to visualize the letters Yud-Hei-Vav-Hei when they pray. You don't have to do this either. You don't have to visualize anything for HaShem - not the words, not the letters, not any image. Nothing. What you should do instead is think about what you're actually saying. When you say HaShem's name in a blessing, think about the meaning - that He is the Master of all, that He was, He is, and He will be. You can say this silently in your mind: Hayah Hoveh VeYihyeh. Don't say it out loud because you can't break up the prayer, just think it. You can also think about all the great things HaShem gives you - the air in your lungs, your vision, your hearing, your food, your family. Focus on gratitude for His gifts and His actions, not on trying to picture Him physically. The more grateful you are to HaKadosh Baruch Hu, the easier it is to connect to Him in prayer. If you want to look at something while praying, look at your siddur and the name of HaShem written there. This is like His billboard - it's not that HaShem looks like letters, but that's where you know to direct your prayer. It's pointing you to the right destination. But never look up at the sky trying to see something physical - that's a mistake many people make. The main thing is to focus on who you're talking to, not on your problems. Think about HaShem Himself, recognize His significance, and pray like a beggar with humility. That's what proper prayer requires.