If you have Content Management on your site, then you will often have the ability to upload an image to the site. In order to upload your images to your site, you should first re-size them to a web-friendly size.
By making the images a smaller size, it will both speed the time it takes you to upload them to your website and also make your website faster. This can be done using standard editing software which often comes with your computer, camera or printer. Alternatively you can use a free program such as Irfanview
The images uploaded to your site should not be more than 800 pixels wide. Most digital cameras, for instance, will take photos which may be upwards of 3000 pixels so are far larger than required for a website.