For a general guide to get started, consult University of Reading's Understanding Digital Images.
When creating your own digital image scans for use in PowerPoint or another presentation tool, these are the general guidelines we recommend:
Scan the image at 300dpi and save it as a TIFF file. This is your "archival" copy from which you can create smaller images to use in presentations. Do not load TIFF files into PowerPoint or other presentation software program. Open the file in an image-editing software program such as Photoshop and resize the image so it is 1024pxls on the long side. Save as a JPEG. Now the image can be loaded into PowerPoint, ARTstor's On-Line Image Viewer or other presentation tool.
If you want to make additional slides of details, go back to your original TIFF file and create the detail from that scan.
For additional information on standards for digitizing various materials including audio and video, consult the CDL Guidelines.