I will share my thought process behind how I make my thumbnails! (I may have overshared a bit..........)
As you can tell, an amazing thumbnail needs alot of thought and effort. I hope that my tips will help you improve your thumbnails at least a little bit!

I'll use my own levels as examples alot, so make sure you're familiar with their thumbnails: bopimo.com/users/Gojab1111/levels

=== Point of interest!

The first thing you need is a standout part of the level that'll look really cool. It should look pretty and ALSO communicate the focus of the level.

Examples:
bVoid and Flow Like Fire are focused on the obstacle course gameplay, so I picked standout parts of gameplay. The thumbnails also match the vibe of the levels - Flow Like Fire is kinda noisy, intense, and compact just like when you speedrun it. bVoid's is mysterious and abstract - surreal just like the rest of the level.


Don't be afraid to make "custom thumbnails!"
For Microbop Flight Simulator and Invasion of the Shadows, to more effectively communicate the flying gameplay / the story focus, I basically disregarded the actual level in favor of communicating a message with a custom picture.
In other words: I modified the levels to make a little section JUST for the thumbnail.

Even for Debounce Chambers, a gameplay-focused level, the thumbnail is "custom" (It's not a playable part of the level) because I wanted to really focus on the springs and interiorness of the level, and couldn't find a good spot in the main level to show those off.


Unfortunately, if your level is a floating obby without much going on, it'll be hard to make an interesting thumbnail.


=== Framing!

Very important! This is what differentiates thumbnails from in-game screenshots.
These tips apply to photography in general - you might want to learn about the rule of thirds, keeping a visual balance, guiding the viewer's looking direction ect.

- Zoom in or out using a good balance of FOV and physical distance. Lowering the FOV usually looks more cinematic, but going too far can also make it harder to perceive depth, so sometimes standing back a bit is the better solution
- Pick an angle and zoom that will leave out any distractions like floating platforms on the edge of the screen
- Manipulate the render distance and fog to get rid of distracting backgrounds
- Don't be afraid to modify the level to improve the thumbnail's composition / remove distractions. You can always revert your changes and keep the thumbnail.
- Don't leave too much empty space on one side of the thumbnail (pick a different angle or add decoration to make sure the screen isn't too empty)
- Pick a dynamic angle!! Looking straight ahead at a build from ground-level is incredibly boring. (alot of levels do this) You have Noclip! Fly!!
- Thumbnails are quite small! Make sure everything is visible from afar.

Examples:
- Microbop Flight Simulator has the shooting star on the top right to visually balance out the purple ring on the bottom left
- Flow Like Fire's structure is framed to take up the entire thumbnail's size, leaving little empty space on the edge. The diagonal angle also makes it look more dynamic and lets you see more of the structure.
- The Climb's camera angle is perfectly placed so that the sky's line things fill the space next to the tower!

Whenever you take a photo, make sure to look at the little preview to make sure the framing is EXACTLY the right size. (the outline overlay when you take the picture is inaccurate!)

=== Colors!

This applies to the whole level, too...
- Pick nice colors that go well together
- Avoid too strong contrasts that take away attention from the main focus (Example: To The Party on the Sun's shooting star is very dark and distracting! The Climb's yellow structure at the bottom also takes up alot of screen space and takes focus away from the tower.)

=== Good Examples!

- Bopimo! Tutorial (the featured one) Has a serene and welcoming atmosphere, which a tutorial should be. The fog shoftens the background mountains' color and gives space for the foreground to breathe.
- Bloodthorn's Grand Slot Machine is simple yet very effective! The colors, framing, focus on the level's gimmick, it's all there!!
- Casa De Galunga look massive, infernal, and imposing, just like the level. It focuses on one tower, and the rest of the screen is filled with evil and fire. It could've used more zoom but that's it!
- Boochin in general has (had?) many great thumbnails!!


=== Counterexamples!

No hate to any of the level creators here - I'm just pointing out ways the thumbnail can be improved with concrete examples!

- chill rooftop night's thumbnail is fine, but looking straight ahead into nothing is boring. I think having the camera focused on a Bopi walking around this big city would communicate the "chill" midnight walk idea even more!
- Journey to the Star Parade! is beautiful, but the thumbnail encompasses too much so you don't really focus on anything in particular. It also doesn't showoff the main gimmick very well!
- Clover-Field and Edge cases. have beautiful scenery that encapsulate the level pretty well, but the composition is just... looking ahead... in a very uninteresting manner...


For my own levels...
- MESMERIZER could've really used a "custom thumbnail" - modifying the level to make the scene smaller would help make Teto and Miku more visible and cut out the random junk near the edges.
- Debounce Chambers! is kinda basic. It conveys the chaos well enough, but not how the debouncer springs work, which is pretty important!
- My first level does like just about everything wrong when it comes to my thumbnailography advice ^^