I mostly use Google image search trying to be careful that the images I use are of good quality, freely available and, if necessary, appropriately referenced. How I make the individual choices is of course part of the creative process but I thought I'd share a few simple steps that have helped me in recent presentations.
The legal side of this is that one should ONLY use images that are available for reuse. Google can help you there. Let's work through an example.
I want to illustrate something with ice cream.
Got to Google and simply enter "ice cream". You should get
There are 3 very helpful buttons. No1 will take you directly to images of "ice cream" with some (occasionally) helpful collections across the top. Before you get carried away (there's a lot of lovely ice cream) you need to filter the search using first the Search tools button No2, then the Size option selecting LARGE. Then under Usage Rights (No3) make your choice of appropriate options. THIS is the group from which you should chose your image.
I then scroll through until I find one that I believe fits my purpose and click on it. Go to the individual image, make sure it is as zoomed in as possible by clicking on it and then save by right clicking. Then how you use the image within your presentation depends on your composition.