Answer:
Yes, Its a good idea but depends upon type of image. Like adding a logo to all the slides at top left corner is not just good but a must for all as well, when we are preparing official documents. However, other images when decided to be repeated must convey the correct intent in each page. If this is not the case, you need to avoid it. Intent is the most important aspect certainly. For rest see explanation.
Explanation:
Yes, and as mentioned above adding logo and other images meaningfully, and with correct intent is a good idea.
However, images must not be blurred, tiny or stretched. Also proper cropping is a must. The border should be solid as well. And when selecting a lot of images, the images must convey the correct meaning together. Moreover, the aspect ratio after stretching must be same as the original. You can do this in PowerPoint by going to format > crop > fill.
Ensure that watermarks are not revealed. Buy them rather than copy from somewhere for better outcome. And never amputate your readers with a headache creating backgrounds. You will loose readers then. The clip arts must be used in right number, and not a lot of them certainly.
Ensure both text and images are in correct ratios. Avoid using more than 2 images per slide.
