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. Colours is a very comlex field. The same picture may look different on different monitors and also on different operative system. Browsing the web you often find very dark picture, it is very likely that the pictures were produce on a Mac.