The law allows "fair use" - so when reviewing a videogame, you can use a single screenshot and it will represent 1 second of the game and is deemed fair use.

Similarly with writing, you can quote a small snippet the size of those blurbs you see in the search results, and it is deemed Fair Use. If you quote the whole thing without permission, it is THEFT under the law.

And with photos - by definition you are using 100% of the original creator's image, unless you are only posting a snippet of the picture from the top corner. Under the law, this is theft, which means that the owner has the right to pursue you and force you to pay damages.