The current texture is the image that will be operated on by operations in the Tools Menu. The current texture is displayed with a blue square around it. The other textures have gray squares around them. To change the current texture, click on another texture.
The size of the current texture is displayed in the Selected: list at the top of the window. If you select another size of texture in this list, you will change the current texture also. This can be useful for selecting very small textures, such as 2 x 2 or smaller.