Possibly because some pictures are placed in line with the text, while others are behind text.

Try going to the Layout tab on the ribbon, and use Selection Pane towards the right. A panel will open up to the right, and it will show you all the objects (pictures and other) that are on that page. You may have to click on a few to figure out which is which - for example, with three images on a page it may say Picture 1, Picture 5, Picture 11. They aren't necessarily in that order. When you are able to select the item you can't click on normally, click on Picture Format, which should appear at the end of your ribbon, and check the Wrap Text features for that item compared to the others. See if you can select it better if it's "In Line With Text."