You can create a custom folder by moving to the Directories tab, the third tab down, below the Parts tab. The Insufficient Storage Space error occurs when you have more than five megabytes of texture stored on your account, as this is the amount given to each user for free. Even when you delete parts, the associated textures are still stored and must be deleted separately. You can manage your textures from the Textures tab. the second tab from the bottom, with the checkered square icon. Hopefully this helps you; feel free to ask any other questions you have.