Usually scanning software can lock the aspect ratio. When you choose your picture size also think about how big you want the picture to appear on the monitor. Personally I like to use 600 horizontal pixels for quality pictures, and 200 or 300 pixels for publishing on the web. I find it important to be consistant, so you do not have pictures of different width on the same page, at least not too many widths. Different picture higth one the same page I find ok.
Quadratic pictures are also beautiful.
No point in scannning better than 96 dpi the monitor can not resolve the extra resolution. Picture format For displaying on the monitor I prefer to save my files in the Jpeg format. Compression is ok and the format is supported by all internet browsers. I usually never go lower than 3, below the pictures clearly drop in quality. Colours Save file using RGB mode.