Finding the right size for an image is an important skill to develop. For the most part, you want your images to be fairly small; 150x113 is a good average size. However, most images that you find aren't the size that you want for the web. You must resize the image, and there are a few different ways of doing so:

Use Plone:

Newer versions of Plone (2.1 and higher) have a built in image scaling feature that supports some preset image sizes for you to use. This is great because you can upload images of various sizes, then size them up or down. That means you can use the same image with different sizes. When you insert an image into a page you can access the resizing options with this drop-down menu:

Use Photoshop:

Adobe Photoshop is a powerful image manipulation program. Watch this video on resizing with Photoshop.

Other Software:

There are other programs besides Photoshop that work perfectly well for resizing images. XNview is a free, open-source software tool that you can download and start using right away. There is also Paint, which comes with Windows and is capabale of very basic resizing. Corel Draw is yet another option. Since all of these work differently, you should read the help documentation to find out how resizing works in your chosen program.