How to Disable Automatic Picture Compression in PowerPoint 2010
If you are working with high quality images in PowerPoint and your images are compressed automatically, you may want to turn off the automatic compression for pictures and images in PowerPoint 2010. This way you can prevent the images to being compressed.
In order to prevent PowerPoint to compress images in file you need to open Options (File -> Options) and then look for Advanced tab. Here look for Do not compress images in file and uncheck it. Checking this option provides maximum image output quality but the output file can be too large. If the option is checked then the images in the PPT file will be down-sampled to the target output specified below in pixels per inche.
Alternatively you may consider to check how to compress images in PowerPoint if you have the opposite problem to solve.