\begin{tikzpicture}
	\onslide<1->{\node at (4.0,-0.0) (figa)  { \includegraphics[scale=0.3]{images/2_4_1.png}};
		\node (texta) [below right = 1mm and -3mm of figa, anchor=east]{\tiny \textsf{Backpropagation}};}
	\onslide<2->{\node at (4,-3.5) (figb)  { \includegraphics[scale=0.3]{images/2_4_2.png}};
		\node (textb) [below right = 1mm and 1mm of figb, anchor=east]{\tiny \textsf{Guided Backpropagation}};}
\end{tikzpicture}