For posting pictures on the web or this site - what is the best dpi or compression ratio

For posting to the web, DPI is totally irrelevant. As for compression, I try to keep my JPEG images close to 100k, no bigger than 200k, in filesize for Model Mayhem. I also have most of my images sized to 500 x 752 pixels. If I were to upload a different size of image, the filesize would naturally be different as well. I also use the "save for web" feature of Photoshop to keep an eye on filesize, preview color balance and gamma for both PC and Mac, etc.