Save as a jpg compressed. 90 or 100 dpi if that's a choice. But mostly resize the image to between 640 x 480 - 1280 x 960 and save as a JPG

What you are getting from the scanner might be BMP format and will be very large. The monitor and the web can't do better than about 72 DPI and recognize any difference.

In general the only three file type that display on the web are JPG, GIF and BMP.

GIF is graphic interchange format. Useful for icons and small files. They can also be created with transparent backgrounds and as moving animations.

BMP is an uncompressed image formate, large, but doesn't lose quality through various edits.

JPG is a compressed image (which you can select the amount of compression) I often use 80% for web images. The advantage is size, but if you open, edit and close, then open, edit close a few times, you can start to see deterioration.

Short answer. Save the image as a JPG.