Compose for a lock screen, not a poster
A phone wallpaper has to survive icons, a clock, and notifications. Keep the character in the lower third or off to one side, and leave the top area calm. A busy sky or a detailed cityscape behind the clock will make the screen hard to read.
Vertical 9:16 is the default. If you need a tablet or desktop wallpaper, generate a separate composition rather than cropping a phone image; the character will be too large or the negative space will disappear.
Pick a mood and a limited palette
A dreamy night sky, a soft autumn palette, or a clean pastel gradient all work. Limit the palette to two or three colors and one accent. The character should remain the focal point even when the screen is dimmed.
Avoid small repeating details near the top of the image. Stars, sparkles, and tiny leaves can look like visual noise behind app icons. If you want texture, keep it low-contrast and away from the clock area.
Test it on the actual screen
Set the image as your lock screen and look at it with notifications visible. Check whether the character’s face is covered, whether the clock is readable, and whether the image feels too busy. A wallpaper that looks great in a gallery can fail on a real phone.
If the character is too large, regenerate with “small character in the lower third, lots of negative space.” If the colors are too strong, ask for “muted pastel palette, low contrast in the upper half.” Small adjustments make a big difference at screen size.