I have two images approximately the same size (800KB) but only one image is being exported as shown in the web capture.

To be honest that image tells us nothing, (as in where the image that is not being exported, is supposed to be)

The image is in the images file, and it's suppossed to be where it says "Placeholder picture"

An image doesn't reveal if you have written the path to the image correctly, if the image name is spelt correctly, or if the missing image has been uploaded with the rest of the page. To be of any help we would need to see the code. The best is if you could let us have a look at your project file.