To optimise and image for the web you can scale the image (reduce the size) and reduce the quality as this is often not noticeable on a computer screen.

Scaling your image

1. Open an image in your GIMP application
2. Click Image from the main menu
3. Click Scale Image. The Scale Image is displayed:
4. Change the Width and/or Height of the image. To constrain the proportions click the link link to the right of the dimensions. Select the measurement type from the dropdown box, i.e. inches, pixels, etc...
5. Click Scale

Optisimising your Image

1. Click File | Save As
2. Enter the Name of the file.
3. Select the folder to save the file in.
4. Select the File Type to save as. The default is JPEG and this is best for photos. PNG file types are best for transparent images - JPEG does not recognise transparency in an image.
5. For JPEG images the following screen is displayed: Use the sliding scale to adjust the Quality and therefore the size of the image. Make sure the checkbox to show preview in image window is checked so you can see the how the image will look.You are aiming to have your images less than 100kb with an ideal limit of 50kb.