Images in the new templates are responsive by default. That means they will resize to fit their containers.
This image is 767 pixels x 263 pixels, but because it is responsive by default, it will resize to it's 4 column container.
In rare cases, you might need an image to not be responsive. An image map is an example of such a case. This is the same image with the class
img-non-responsive added to it. See how it runs off the page because it has not resized to its container.
Images in addons
Images will resize to match their containers. If you place an image directly into a two or three column addon, it will scale down to fit the addon column.
Note that images will scale down smaller to fit their container, but they will not scale up to fill a larger container. They will stop scaling when they hit their original size. For instance, the three column images will not fill the width of a high resolution cell phone screen because the original image is only 255 pixels wide.
If you need to change the size of an image, free online tools like http://www.picresize.com/ let you crop, resize and more.
If you need to reduce the file size of an image, free online tools like http://optimizilla.com optimizing images for the Web.