To modify a specific element on every slide in a PowerPoint presentation and add an image to conceal something using the Slide Master view, adhere to these steps [1][2][5]:
1. Open your PowerPoint presentation.
2. Go to the `View` tab in the PowerPoint ribbon.
3. Click on `Slide Master`. This will take you to the Slide Master view, where you can make changes that will apply to all slides in the presentation.
4. In the Slide Master view, you will see a list of slide layouts on the left side. These represent the different types of slides in your presentation, such as Title Slide, Title and Content, and Blank Slide.
5. Click on the slide layout that you want to modify. This will become the master layout for that type of slide. If you want to make changes to all slide types, you can click on the first one (the topmost slide).
6. Make the changes you want to the master slide. You can change fonts, colors, background, add shapes, or insert pictures. To add a picture to cover something on every slide, follow these steps:
* Click the `Insert` tab in the ribbon.
* Click `Pictures` to insert an image from your computer.
* Select the image you want to insert and click `Insert`.
* Resize and position the image to cover the specific part of the slide you want to hide or modify.
7. Once you've made all the desired changes to the Slide Master, you can close the Slide Master view by clicking the `Slide Master` tab in the ribbon and then clicking `Close Master View`.
8. Your changes will be applied to all slides in your presentation. If you need to make further adjustments, you can always go back to the Slide Master view and edit the master slide.
9. Save your PowerPoint presentation.
Keep in mind that adjustments made within the Slide Master view will impact all slides using the chosen slide layout. If you require distinct alterations for particular slides, you can make those adjustments individually in the regular slide view after applying the master layout.

To add a picture as a watermark or background to every slide, follow these steps:
1. Open your PowerPoint presentation
2. Access the Slide Master view
3. Insert the picture
* In the Slide Master view, you'll see a large slide thumbnail at the top, which represents the master slide for your presentation.
* Click on the `Insert` tab in the ribbon.
* Choose `Picture` or Picture from File, depending on where your desired watermark image is located.
4. Select the picture and insert it: Locate and select the picture you want to use as a watermark or background. Click the `Insert` button.
5. Position and format the picture: Resize and position the picture as desired on the master slide. You can drag it to the background or adjust its size to cover the entire slide.
6. Adjust transparency (optional): Select the picture, go to the `Format` tab on the ribbon, and use the `Transparency` slider to adjust the transparency level. In PowerPoint 2013 and earlier versions: Right-click on the picture, choose `Format Picture`, go to the `Picture` tab, and adjust the `Transparency` slider.
7. Save your changes: After you've positioned and formatted the picture, you can go ahead and close the Slide Master view. Save your presentation to apply the changes to all slides.
Now, the picture you inserted in the Slide Master view will serve as a watermark or background for every slide in your PowerPoint presentation. You can edit or remove it from the Slide Master view at any time if needed, and the changes will be reflected throughout your entire presentation.

To change one image into another on a slide, follow these steps:
1. Open your PowerPoint presentation: Launch Microsoft PowerPoint and open the presentation containing the slide with the image you want to replace.
2. Access the Slide Master view.
3. Locate and select the image:
* In the Slide Master view, you'll see a large slide thumbnail at the top, which represents the master slide for your presentation.
* Browse through the slides on the left pane to find the specific slide that contains the image you want to replace. This may require scrolling down if your presentation has multiple slide masters.
4. Replace the image: After selecting the image, press the `Delete` key on your keyboard to remove the existing image from the master slide. Now, go to the `Insert` tab in the ribbon.
5. Insert the new image:
* Choose `Picture` or `Picture from File`, depending on where your new image is located.
* Locate and select the new image you want to insert to replace the old one.
* Click the `Insert` button.
6. Position and format the new image: Resize and position the new image as needed on the master slide to replace the old image.
7. Exit Slide Master View: Once you've replaced the image, you can exit the Slide Master view. Click on the `Slide Master` tab in the ribbon. Select `Close Master View` to return to your regular slide view.
8. Review Your Presentation: Check your presentation to ensure that the old image has been replaced with the new one on the specific slide.
9. Save your changes: Close the Slide Master view. You will be prompted to apply the changes to all slides. Click `Yes` or `Apply to All` to replace the image on all slides.
Now, the old image on the master slide will be replaced with the new image, and this change will be reflected on all slides in your PowerPoint presentation. Make sure to save your presentation to preserve these changes.

Please be aware that the specific steps might vary slightly based on the version of PowerPoint you are using. These instructions offer a general framework for achieving the desired outcomes in most PowerPoint versions you are using [1][2][4][5][6].