SUPER awesome question(s)! I wanted to do this recently while I was setting up a Sheet to help me brainstorm and track the components of a Breakout EDU game I was writing.
In my example, I wanted to be able to sort the sheet with the elements I was using, but the images would't play along until I figured out the answer to the very question you're asking.
So first of all, you need to get the correct kind of image link. You just need any image that has an image extension at the end. JPG, PNG... any of those will work as long as you see it there. You can also hone this in while performing your search by typing in "river otter type:png" or "river otter type:jpg." Regardless of your search method, when I search for an image, I click "View Image," the verify that it has what I need.
The URL I'm looking for looks like this:
On the other hand, if I want to use an image I created, I just upload it to a hosting site like imgur.com.
Once you have the direct link to the image URL, the formula you will need to insert in the spreadsheet cell is:
=image("image link goes here")
Be sure that the link ends with the proper extension and that you enclose it in both quotes AND parentheses.