By default when you save a Layer Style as a Preset from Dresser it wil generate a 100x100 png-file as a thumbnail besides your preset.
You can manually change this thumbnail by replacing this image with another image that has the exact same name as the preset like so:
Preset: [name].ffx
Thumbnail: [name].png
But you can also create them through the Preset Menu of the Preset itself.
right click your preset and/or open up the Preset Menu
<aside> <img src="https://s3-us-west-2.amazonaws.com/secure.notion-static.com/20180913-bb60-461c-abf3-caef0e9da7df/Smock_ImageAutoMode_18_N.svg" alt="https://s3-us-west-2.amazonaws.com/secure.notion-static.com/20180913-bb60-461c-abf3-caef0e9da7df/Smock_ImageAutoMode_18_N.svg" width="40px" /> Create Thumbnail Creates an automatically generated (transparent) thumbnail from the preset of 100x100
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<aside> <img src="https://s3-us-west-2.amazonaws.com/secure.notion-static.com/098a50ee-ec15-4714-bd1e-ea6ce822f92b/Smock_ImageNext_18_N.svg" alt="https://s3-us-west-2.amazonaws.com/secure.notion-static.com/098a50ee-ec15-4714-bd1e-ea6ce822f92b/Smock_ImageNext_18_N.svg" width="40px" /> Create Thumbnail from Current Frame Creates a thumbnail from the current frame of your active composition of 100px height
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<aside> <img src="/icons/compass_orange.svg" alt="/icons/compass_orange.svg" width="40px" />
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