When designing YouTube thumbnail, you need to consider a few things.
Let’s see those in detail.
Relevant image
Never use images not relevant to your video content. If you use attractive pictures to bait audiences, viewers will definitely click on your video.
You see the spike in CTR(Click-through rate). Once your viewers found it’s not relevant they simply drop off. It’ll highly impact your audience-retention rate and YouTube decreases your video ranks for that keyword.
Image format
Use PNG format and add the target keyword on the thumbnail image as well as in the filename. PNG format is highly recommended because it stores the arbitrary text strings together with the image.
So YouTube can easily read the text from the thumbnail image.
Image size should be less than 2MB. I personally use Snappa for its simplicity and contain more visual elements for free.
Thumbnail Background color
Your Thumbnail should stand out from the crowd. Use Orange, Blue, Yellow, and Green as a background/frame color because the YouTube interface is made of red, black, and white.
Readable text
Text on the thumbnail should be readable by the viewers who watch your videos from a different platform(Mobile, desktop, tablet).
So pay close attention to font type and size.
It should be completely contrasting to your background.
It should not be hidden behind the YouTube element. Increase the font-size and limit the number of characters to 20 to stop text overpower the visuals.
For e.g., if you add text on the down-right it’ll be hidden behind the YouTube watch time on mobile.
Have a look at the second video bottom line text.