Traditionally, things like scales, glasses, trousers have been seen as coming in two's and therefore are always used in the plural form.
Nowadays, "scales" can be made from a single part. Actually, most often they are a single part, like in your first picture. But grammatically, we don't care about that. They are plural scales. Just checked the amazon website, and all their weighing equipment is sold as "scales". You wouldn't say "a pair of scales" for one-part scales, but they are scales.
So I would say that "Use a pair of scales" and "Use a scales" or "Use a scale" would all be wrong. "Use scales" would be right - unless the person has an actual pair of scales like your second picture.
(Things get funny if you have more than one scales. For example, I bought two each of the items on your two pictures, four items in total, and you ask me what I bought... I'd answer "two scales and two pairs of scales" but have no idea if that would be correct. Just avoid doing this :-)