What webpage software are you using?
For instance WebEasy, which I use - it's just "insert image" during the page layout phase - hold down the Ctrl key as you drag and place the image, and a thumbnail is automatically generated - which will pop-up to a full size when clicked on. Couldn't be simpler. But it varies from software to software how you would do this.
Best dpi for websites is usually 72 (that's what most monitors used to view pictures work at.) The idea is to keep the file size down - which also means you should convert them to jpgs before inserting them into the webpage.