Digital photographs can be huge in terms of file size (3 to 4 megabytes, for example). Those large file sizes contribute to an inflated presentation file size, too. However, PowerPoint 2007 has ways to keep your file size manageable.
One is automatic picture compression. When you insert a photo, PowerPoint reduces its number of pixels. In high-resolution (meaning high-pixel-count) images such as photos, there's often pixel data that you can lose while still retaining fidelity in the picture. So, the image doesn't look different in the projected presentation, but its file size is smaller.
In addition to automatic compression, you can use the Compress Pictures button, on the Format tab, to apply more compression. This further reduces the pixel count per inch. The Screen option (150 ppi, or pixels per inch), as shown above, is the optimal choice for projection.
Do you want more control? You may prefer to open your photos in a photo editor before you insert them into PowerPoint and determine their compression settings yourself. In that case, you can turn off PowerPoint's automatic compression. Steps for doing this are given in the Quick Reference Card.