Image Optimization
not many people seem to know this

Hate it when people send bloated presentation attachments. Download 15-20 MB and all you get are 3-4 slides with a few images.
Amazing how you want all the cool presentation tools & tips but do not know how to manage design information.
Simple rule is, what you need to see on screen does not need to be print quality. Forget print quality, at 100 dpi screen resolution, you won't be able to appreciate resolution beyond a point.

The method to be used to get your image sizes down is known as optimization. One can use an image editor like Fireworks or Photoshop, or any other image editor.
Step1- Get the image size scaled down while maintaining the proportion. An image width of 800-900 pixels would fit into most laptop screens. Google, facebook etc page layouts are mostly around 900 pixels wide. Work with a smaller size if it's to be inserted in a slide.
Step2- Export or save as 'gif' or 'jpeg'. . . GIF OR JPEG is the question.

Your un-optimized file formats are like PSD, PNG, BMP, TIFF. Any of these files can be saved as GIF or JPEG.

GIF and JPEG are optimized file formats.
GIF is designed to handle images with solid blocks of colours. Like below:

So images with solid blocks of colours (like maps and diagrams) are to be converted to Gif file format. This would retain the image quality better and also reduce the file size better.

JPEG is designed to handle tonality. Like below:

Images with tonality like real life images, photographs, colour with gradients. These type of images are to be converted to jpeg file format.

Image optimizers let you choose jpeg quality in terms of percentage. Mostly 65 to 75 percent quality is good for screen view, depending on the original image quality. In case of gif ,one can choose the number of colours to be used to define the colour block. 256, 116, 64, 32, 16 and so on. However the shade will also change slightly as you reduce the number of colours. The file size of-course will also come down.

An image with solid block of colour when converted to jpeg will show blotches as the image quality is brought down. An image with tonality, when converted to gif will show banding to define the gradient.

A note about jpeg. It is a lossy file format... which means that every time you save a jpeg file, it looses a bit of clarity. So the rule is to work in un-optimized file formats and save the final work in a suitable format.