Would I be able to add a picture from the internet if I needed to show the readers say, makeup, so they can compare?

Just wondering, is this allowed?

No problem as long as the images are royalty free, not owned by someone else or otherwise copyright copyright protected.
I many "free" cases like Wikipedia it is no problem to address the issue by stating the image is from the linked Wiki article.
For other images it is advised to check the rights and if in doubt get the consent of the person/company holding the rights on an image.
To be clear and give a bad example:
Taking images from some personal or commercial website is as bad as using images you found in someones Facebook profile.
Similar story if you use images from different sources to make comparisons - without consent this could be seen as you trying to give a bad reputation to a product you don't own - and it can end up costly depending in which part of the world you call yourself home...

UnicornsAndTacos (author)  Downunder35m9 days ago

I'm still a bit confused.