How to:
Update your mod, but leave some of the information unchanged.
If you want to update your mod, but not update certain information on Steam, you can leave some of the fields blank.
Simply point to your mod source folder and remove any fields that you do not intend to update.
How to:
Use a custom thumbnail image instead of a generated.
If you want to be in charge and control of the thumnail image, instead of letting it generate from the mod UI, add to your mod’s package.json the option "image" : 'url/to/your/image/from/root.png" if it’s not already there, and if already available, change the image file to point to your desired one.
Remember to not submit an image throught the publish panel, but instead use Steam Workshop directly to upload and manage the pics that you want. That way, you keep them separated.