I can’t tell if JP is just roleplaying a wizard who doesn’t know his own power or if he hasn’t, himself, got the hang of illusions in D&D. I don’t really care either way, I’m enjoying the show a lot. In any case, there’s a fact that illusions are tricky to use in D&D. They’re like come-backs; you think of the perfect one an hour after the moment it mattered. So I collated here everything I know off the top of my head about using minor and major image. For everyone to use. 
Store in our heads for later.
Imminent Battle Situations:
- Fake outs. Do you think you’re walking into an ambush? Create illusiory allies ahead of you to spring the trap and then counter attack with real allies. Peeking corners as well, you can make a minor illusion of someone coming around a corner and drop their pack. Stick your head/familiar in the pack.
- Make your own ambush. If you’re being pursued and the encounter is several mooks and 1 big threat, it might be worth it to hide the party inside a false wall or something appropriate and all burst out when the real threat is most vulnerable and burst-dps them down.
- Need to run away? Set this one up with your party before-hand so they know how to behave: On your secret-signal, the walls/ground will appear to grow teeth and eat the party. The teeth then move towards your enemies, likely buying you time.
Battle Situations:
Illusions can be primarily used for action and area denial of your foes by creating illusions of things that your enemies won’t even risk touching or even as just something to block LoS. Here’s some examples of those things:
- -You fling a sparkling bolt from your hands and where it lands it spreads sideways and the ground falls away leaving a great cravasse. For extra points, move the boundaries of an existing pit to convince enemies to jump in.
- Create the illusion of spectral undead coming slowly, reaching out a hand whispering “join meeeeeee”. This may depend on the DM but many are comfortable with ruling that interacting with the illusion doesn’t automatically reveal it if the illusion is of something that can’t be touched.
- You raise your arms and great, spinning blades rise from the earth blocking your enemy’s path (an immitation of the spell Blade Barrier).
- You raise a magical mist that turns and boils which keeps interposing itself between your party and the distant archers. Neat!
- A last minute distraction to buy an ally, or yourself, time. Many people will turn in surprise if there’s suddenly a snarling panther behind them.
- Against sentient creatures it might be appropriate to make illusions of their own kind who call an urgent retreat. Perhaps their base is under attack?
- Against non-sentient creatures you can often attempt to scare them off by making an illusion of a bigger, angrier creature showing up.
(Remember, it’s very important that your intended targets really don’t want to touch the illusion due to how the spells function.) - Last but not least, the old battle-rock. Use minor illusion to make a great big rock, 5ft of wall, a small hill of earth. Anything that obscures you from your enemies. You, or your allies, can peek out from behind it and take pot-shots at your foes. You can even shoot through it but that could very well reveal it as an illusion but even if it does, it only cost a cantrip and they’re pretty cheap to cast.
Social situations:
- Keep a spare, blank piece of paper and always be on the lookout for what any important NPC’s seal, handwriting or signature looks like. You can put illusory writing on the paper. It’s very Dr Who.
- This is a little on the combat side but mostly (anti)social since it involves talking and knowing the right information: Taking illusory hostages. Remember to make more than one if you can. It’s nice to have a spare.
- If you’re going to go a negotiation of the tense kind, stay somewhere you can quickly duck out of sight where nobody will see you perform the somatic components of a spell, have a party member know the signal and make a lot of noise to cover your verbal combonents and suddenly you’ve brought illusory reinforcements.
“Why don’t we just take the gems from you?”
“Because… WE ARE VERY NOISY! … and we brought a squad of crossbowmen with us.”
P.S. Don’t forget when appropriate to have a secret word to let your party know that an illusion is in play so your allies don’t fall for the tricks meant for foes! D:
Happy hunting to us all as illusionists!