The use of clipart in presentations is usually frowned upon, and rightly so. Clipart has a reputation of looking cheap, silly and pixelated. You've probably seen presentations that liberally sprinkeled stick figures and other clipart over the slides in an attempt to make them look less boring. In the end, this approach is yet another case of slides that are neither fish nor fowl.
So, in general, if you want to illustrate your point, using full-screen photos is a better and more effective option.
There are, however, legitimate uses for clipart and that is in diagrams and flowcharts. By which I mean that you often need symbols in your flowcharts, like computers, databases, etc. It's perfectly fine to use clipart for these. Still, you need to be careful in your selection. The symbols should be clear and easy to understand and they should be of high quality - avoid pixelation at all cost.
Where can you find good clipart? A quick internet search for "free clipart" will give you many results to choose from. A website that I often use is Openclipart. It has a huge selection of symbols, many of which are of high quality. Specifically, it provides the clipart in SVG, a vector format. Which means that they are scalable and therefore avoid the pixelated look. You can either download the SVG file or let the website render it as a PNG in the size you need.
Another good source, and this is a tip from one of the participants of my workshop at FrOSCon, is the collection of icons in open source projects such as Gnome. We usually associate "icon" with small 32 by 32 pixel pictures, but in the days of retina displays this has changed. Nowadays icons are often of high quality and high resolution and therefore good for use on slides. As an added bonus, they may even be recognized by your audience as something they've seen on their computer (depending on the topic of your talk and your audience, of course).
As with photos, there's the legal aspect to consider. The use of clipart from Openclipart is free - the images are in the public domain. Icons from open source projects are usually under very liberal licences as well (a licence file is usually included). There are also commercial clipart collections but in most cases, the above-mentioned sources will provide you with everything you'll need.
To summarize: Don't use clipart to "spruce up" your slides; this has never been a good idea and it only makes your slides look crowded and amateurish. For diagrams and especially flowcharts, the careful use of high-quality clipart is perfectly fine.
(Sample slide by yours truly, using clipart from Openclipart)
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