1. Create button 1 and button 2
2. add action in two buttons for variable value (eg. vVar=0 for button 1 and vVar=0 for button 2
3. Create 2 more text box and load image to each text box( Image 1 and 2)
4. Go to image text box properties > Layout> Conditional show
For Image1 Text box-------- if(vVar=1,1,0)
For Image2 Text box-------- if(vVar=0,1,0)
You probably already guessed that there is no conditional show for button images, or an expression field that decides on which image to show. Just like there isn't any support for changing X/Y/H/W-coordinates of charts and other objects based on the outcome of an expression.
Of course, if you don't like the hidden button-approach, you could always try to solve this programmatically using macros. And as always with macros, your client may decide not to support your solution...