Quite often you will hear someone say that images online need to be 72 DPI. There are a lot of (mostly inappropriate) ways of dismissing such silliness.
DPI, or Dots Per Inch, is a printing term that tells the printer how many dots, or pixels, to squeeze into one inch of space. For displaying on-screen, DPI has no relevance. What does matter is the pixel count.
For example, if you have a picture that is 1200 pixels wide and 900 pixels high, you would be able to print it as 4"x3" picture if you set the DPI to 300 (1200 pixels ÷ 300 pixels per inch = 4" and 900 pixels ÷ 300 pixels per inch = 3"). If you set the DPI to 150, then you could make a print that is 8"x6". At 600 DPI, the print would be 2"x1½"
But that's printing and you want to display the images in your web page, right?
Step one, ignore DPI.
What you need to do is change that pixel count. Most LCD monitors today display images at 96 PPI (pixels per inch). So when you import that 1200x900 image, it's going to be really, really large.
Using some image editing program (Photoshop, Photoshop Elements, Paint Shop Pro, Photo Paint, GIMP...) you want to resize the image and change the picture to something like 600 pixels x 450 pixels. When displaying images online, your resolution (the number of pixels displayed in an inch) is set for you and is pretty much standard at 96.
So, when you need to make an image suitable for online use, ignore DPI and, in whichever program you're using, find out how to resize the image.