The forms of the Lord is not imagination. They say that they imagine some form. Sādhakānāṁ hitvārthāya brahmaṇo rūpa-kalpanaḥ. The Māyāvādī philosophers, due to their poor fund of knowledge, they think that, "The Absolute Truth is formless, but because we cannot meditate upon formless, something formless, let us imagine some form." Imagine. Nirviśeṣa-vādī, nirākāra-vādī, they imagine forms.
Therefore their philosophy that, "Any form you like, you can concentrate. Because after all, there is no form. But for your present facility, you can imagine some form." But there is also mistake on their part, because they say: "Imagine any form." Generally, they prescribe the form of Lord Śiva, the form of Lord Viṣṇu, the form of Durgā, the form of the sun and the form of Gaṇeśa. Pañcopāsanā., these five forms. But ultimately, you become "form-less." The example given by them is that you rise up to a upper place with a wooden stair, and as soon as you reach there, you throw it away, so that you'll not be able to come back again.
Similarly, their philosophy is, "With some imagination of form, you worship, and as soon as you realize Brahman, throw it away. No more." These are all imagination; actually there is no fact. They do not know the actual position of spiritual life, therefore they are personally misled, and they mislead others also. Here it is particularly mentioned, liṅgāni viṣṇoḥ, the form of Viṣṇu, not of this rascal imagination of Durgā, Kālī or Lord Śiva.