Hello Mikson, there is an easier way to add a thumbnail for people such as myself who are not too clever with computers. This is a step by step "dummies" guide.
Content Manager, select your Route, "view in Content Manager Plus", click on small square below "File". Select "Thumbnails, select "add a thumbnail" Width is 240. Height is 180. Then click on square with 3 dots next to "new" Select the picture you are wish to use as your thumbnail. When done "save" your work. Go back to Content Manager and go into "edit" as before but this time select "Commit".