I believe in the relevant workflow > arguments tab, you are able to re-order/remove/add "MIME types" and "Thumbnails" renditions. Also you can see the command that is executed to generate these renditions. You should be able to configure your requirements there. Can you verify if the web-enabled Thumbnail is at the top of list in arguments tab ?
Please find the below screenshot .when i upload an asset in DAM its taking the rendition which is configured in Web enabled Rendition tab of DAM update asset workflow . My requirement is when i upload assets in DAM it should take my "customImage.png of size 48*48 " as thumbnail icon while displaying in DAM. Please let me know where exactly i can configure my custom thumbnail icon so that it ll display in DAM.