You can do a Google image search with your keyword & .png or the words 'no background'. Be careful though, many of the results may be mislabeled. True transparent background images have a grey and white checkered background on them in the preview screen.
This image will only blend in with a white background. It is not truly transparent.
The checkered background identifies that this image will blend in with any colored background.
When possible, one high quality image is often just as effective than a collage of pictures.
When pictures with backgrounds must be used consider arranging them in the grid system.
Click the down arrow next to the crop button to give your image a masking shape.
When using side profile pictures of subjects, they should be facing towards the text of your slide. The reason for this is that the audience's gaze should lead from the picture's sight line into your text.