Water spilled into the keyboard of my Macbook - would this affect the quality of the files saved on it?
It's highly unlikely that spilt water on your keyboard will result in your photos becoming pixelated. Instead, it's much more likely that spilt water will damage some of the components of your MBP (depending on what you did immediately after the spill and how long it's been since). 

If your internal drive was damaged, then you'd have all sorts of problems reading from and writing to the drive, which means it wouldn't still work the way it used to (as per your question).

As a test I would copy some of these photos onto a USB drive and try viewing them on another computer just to see if they still look pixelated:


If they do, then you know your data has been corrupted. 
If they don't, then you know you've most likely got another problem with your MBP (e.g. damage to the graphics card, damage to the screen, etc).


If you don't have access to another computer, then email the photos to your smartphone and view them on that.

As an additional step it would be worth testing your hardware.

Testing your hardware

You don't specify your model of MacBook Pro, so it's not clear whether you need to use Apple Hardware Test or Apple Diagnostics to do this. If your MacBook Pro was introduced:


before June 2013, it uses Apple Hardware Test. Follow these steps.
after June 2013, it uses Apple Diagnostics. Follow these steps.