Save your 2 images in your graphics folder.
You can create a parent list (let's call it ImageList) with each image defined as such ...
1 <img src="[%GraphicsPath()%]Image1.png">
2 <img src="[%GraphicsPath()%]Image2.png">
Note: rename the images in 1+2 to your desired image names.
Now create a constructed list (call it ImageConList) and it will use the parent list ImageList ...
This will randomise your 2 images.
When you want to display your image in question script, simple use something like this ...
Please rate the following image: [%ListLabel(ImageConList,1)%]
The first image in the randomised constructed list will be displayed in your question script.
You can export the constructed list in your data and the 1st item in this list will represent the displayed image.
If you want to go a little more fancy, create a variable like ImageDisplayed in your Pass-In fields and define it as a whole number.
Pop this into any of your questions in the footer (make sure the question isn't one that is skipped over) ...