If it seems like your PowerPoint files have gotten very heavy & extremely slow to publish, then compressing the images might help. An image can really add up the memory usage fast. There is a function in PowerPoint that can allow you to compress your images.
By setting PowerPoint to compress your pictures, you can optimize your presentations more efficiently. With this decreased size, you can email them and transport them more easily.
Here is how to decrease the sizes in PowerPoint 2010:
If you select a picture on a slide, then click the Format tab (the one that says “Picture Tools”), you’ll see an option to “Compress Pictures”: