Only Use High Resolution Images

When a potential client goes to your view your portfolio, they expect to see high resolution, professional looking images. So give them that Dont display full size images at 300px by 300px. The viewer is there to see your work at its greatest quality, not a low quality optimized for bandwidth. When you consider your target demographic, chances are high that they are located near you. By now, most of us have high speed cable or DSL connections, so utilize them.
Provide high resolution images for your clients and in return they may have to wait a couple seconds longer. If theyre that impatient that they wont wait for your portfolio to load the best quality possible, then personally speaking, theyre too impatient to be my client.