You consider removing images from the dashboard designer image gallery because the images are old or not used anymore.
Example image in the gallery:
UIM 20.4, 23.4
Dashboard designer image gallery
The Images are kept in the UIM database, in a table called ump_media :
You can easily remove them by using the SQL DELETE statements.
How to use SQL DELETE statements:
https://www.w3schools.com/sql/sql_delete.asp