If you're new to Keynote, Apple's answer to Microsoft's PowerPoint, you may be unfamiliar with both the interface and the graphics editing tools. Keynote offers a wide variety of options in the types of images you can import into your presentation. Once you place your images into your slides, you'll no doubt want to edit or adjust them. Perform this function through the "Adjust Image" window, which offers many options.
Open a Keynote presentation, and then scroll to the slide that has the image you want to adjust. Click on the image on the slide. The image must remain selected, as indicated by the handles around the perimeter of the image.
Click "View" from the menu. Click "Show Adjust Image" to bring up the "Adjust Image" palette.
Drag the slider to the right to increase the value of your adjustments, and to the left to decrease the value. Adjust the image's brightness and contrast by dragging the "Brightness" or "Contrast" slider. Change the density of the color with the "Saturation" slider. Give the image a cooler or warmer hue with the "Temperature" slider. Change the overall tint of the image with the "Tint" slider. Increase the sharpness of or blur the image with the "Sharpness" slider. Increase or decrease the image's level of exposure with the "Exposure" slider. Adjust the levels of the shadows and highlights with the "Levels" slider.
Click the "Enhance" button to automatically enhance the richness of the colors. Click "Reset Image" to ignore all the adjustments you have made to the image.