My bet is that you've got scrambled color reproduction settings -- either within InDesign itself, or if the images were manipulated with Photoshop, in the photos themselves. By my experience this generally results when images are set up for, say, newsprint reproduction but the job (and/or InDesign file) is prepped for magazine reproduction.
Do the colors print as washed out? Or do they just appear that way onscreen?
not sure if I clicked something that is applying some sort of weird fade or washed-out effect to all the images
It looks like you have applied some transparency amount or an Effect to the images or their frames. Open the Flattener Preview panel and choose Transparent Objects from the Highlight pop-up. That will highlight any transparencies applied to objects on the page and you can use the Effects panel to remove effects or change opacity.
If you were to inadvertently set the Effects Opacity setting to less than 100% with nothing selected, then all future rectangles you create would have that opacity setting applied. You could also have an Opacity change applied to a default Object Style.
It will help us analyse better if you click on one of these two images, grab a screenshot of your InDesign "Control Panel".
I am a very experienced InDesign user and am having this exact same problem all of the sudden. I have not accidentally applied any effects or anything. It will happen sometimes just from resizing an image. Can anyone help?