An Image is a CPU-accessible representation of a picture. It is basically and array of colors, one color per pixel, with associated metadata such as width, height, and pixel format. When the width and height of an Image is changed, the amount of memory it requires changes as well.
A Texture is a GPU (graphics card) accessible representation of a picture. The memory layout of this is hardware specific and it cannot be read by the CPU. Textures can be stretched by changing their width and height; the amount of memory it requires does not change in this case.
In order to render an Image in Altus, it must be copied to the GPU, which means a Texture must be created from the Image.