I need to change the file size of an image I created in Photoshop without changing the dimensions for a banner ad on the web. How do I do this?
To add to Ryan's answer, it also depends on the image type, whether to use JPG or PNG. Typically, anything with a photograph in it is best saved as a JPG, to give you the best filesize:quality ratio. Designs without photos in them (logos, typography, icons, etc.) are best saved as PNG/GIF.
Click on the 2-Up option in the Save For Web panel. This allows you to view the original version vs. the one you are saving for web.
Configure your options, keeping an eye on the quality of the image in the preview window. Reduce the colors in a PNG/GIF and the quality in a JPG, until you have a good balance between small file size and quality. You can also try playing with some of other options in this panel, to further reduce file size and improve appearance.
Keep an eye on your estimated file size in the bottom left-hand corner of the window.
You can sometimes get away with JPG qualities of 40-50 and PNG/GIF colors of 4, but it will be different for every image you save, so it involves a lot of trial and error, to find your balance.
To not only help you with more than just getting your file-size issue addressed, but also help you improve your banner production method overall. I've created a template for you using the new art boards feature in Photoshop.
https://choiceomg.com/bat.psd (Click to download my .PSD template)
After you design your banners, go to: File -> Export -> Export As
This will allow you to export your banners with full control over quality, file-size, dimensions, and file type.