ok so first you have ur map.
then you make some thumbnail part.
(VERY IMPORTANT) you take a screenshot on any like photo software (VERY IMPORTANT)
1: steam screenshots (ok but if you enlarge it or make it smaller, it turns a lot more pixely but thats since i use google slides)
(you can see the pixels on it)

2: xbox game bar screenshots (i would reccomend you can enlarge the screenshot and it still looks great on google slides)

ok now ur done with that, you can customize it to your wants, "the skys the limit!" example:

ok so when u finish editing it take a pic of it (Reccomend google slides since you can present and take pic without other stuff) example:
now go on aw3 and go to upload; click the clock here to change thumbnail
then go click on the house button and then you can find the file and upload the battle

now u have learned how to make custom thumbnails. Go!