When designing for the web, it’s important that all your images are in a compressed format. Not so much that your images become pixelated, but as much as possible while retaining quality.
When designing for the screen you’ll need to save your images at 72dpi. Programs like Adobe Photoshop have a “Save for Web” feature that will automatically convert your image to 72dpi if you forgot, as well as give you a variety of compression settings when saving your images. For web this will likely be either png, jpg or gif depending on the particular usage.
