Using an image program like Photoshop or Paint Shop Pro you need to reduce the number of colors and DPI while maintaining as much of the clarity as possible.
Here's what steps will help.
Open the image you want to change.
Check the 'Count of Colors used' in the tool you're using. If more than 256 (8 bit) you'll not be able to do much to reduce colors, but most likely the image has a palette of 17.6 million colors (24 bit)
If a color image, try reducing to 8bit, 10bit or 12bit. If a b&w change it to grey scale and reduce. Some forms can be reduced to 16 (4 bit).
Next, check the DPI, usually in the 'image information' tool. If it's more than 200 try reducing to 200.
Save the image in BMP format with RLE compress if possible. Look at it in the image tool to see if it has held up.
We recently pulled 11 pages from a PDF to PNG, changed them to greyscale BMP at 4bit 200DPI and it reduced the RPT file size from 317000k to 2000k.