custom image sizes possible?

Is it possible to add image size options to the theme? For example, the thumbnail is set at 150x150, but we have certain images we want that size to be 300x150. So we want to keep the current thumbnail setting but add something called thumb-long that will give us another choice for sizes. Alternative, is it possible to modify sizes of individual images on a page. For example, if the large size of the image is 400x600 but we want to make one image show as 300x500 - how can we do it? Thanks very much.