I need your expert advice and opinion regarding the proper image format to use in website, and its recommended resolution and file size.

1. What is the best picture format to use in web pages?

2. What is the maximum image file size that can be uploaded in the webpage?

3. How can we increase the image resolution of the photo to be uploaded in the webpage?

4. Is it possible to have a high resolution image yet it has small file size? if yes, how can i do this in Photoshop?

5. Can i increase the resolution of the image without changing the pixel dimension? For example, fixed 615x215 pixel dimension, then increase the resolution to 300.

6. I read several post that the standard resolution is 72 or 96? is possible to have a higher resolution, say 300? How to achieve this in Photoshop?

More importantlly, does high image file size and resolution affect the Google page ranking and performace on the website?

I really appreciate your comments and advice on the above points. Thanks