I think the easiest way is to copy and paste another existing image/texture icon and then edit the copy by choosing Edit Catalog item under the icon right click menu in the catalog. Browse to the image you want to use instead.
An issue with that is that the icon won't update with the correct image.
Place a texture on a part using the SmartPaint settings on the Part right click menu.
Use the Fill EyeDropper tool under the Visualization ribbon bar to fetch the texture and place it inside the catalog. Now you'll have a new texture icon with the correct settings.