Why do my pictures become gritty when I make a collage?

Hi, I have a Nikon camera that takes really good pictures… I uploaded some and put effects on them with Ribbet, com and they came out beautiful ( you can zoom in with the site and you don't see a pixel)
Long story short, I can't decide between two pictures so I made a mirror-like collage. The problem is it's all grty and it looks like you took the pictures with a potato, it's even worst when you zoom in… Can I fix it? If not, can someone recommend some good sites to do this without this problem?

Sorry if the terms I use are weird, I'm french lol

Don't use online tools, if grade quality of original image is larger or high,
then placing sets into smaller frame, would lower each image to fit frame, and that causes lower quality per image,

the size as mp mega pixel value from camera into pixel count versus the pixel resolution, versus the inches or cm values in size of one image, those placed into collage may make image too large, or shrick each to lower quality,

for online use, you may want to compromise base 72 ppi/ dpi for webpage viewing,
or drag and drop utility for quality in print out only,

you could use paint, but the visual mapping would be as large as your wall in scrolling,

PhotoScape (with collage tool)
http://www.photoscape.org/ps/main/index.php

PhotoFiltre ( as single editor)
http://photofiltre.free.fr/frames_en.htm

check software you have for camera, and for printer

What ever you used to make the collage clearly degraded the photo, some tools do that depending upon how you did it. Also if taking a smaller photo and expanding it larger, that degrades it especially if not a larger photo resolution to begin with (but most today are all plenty high resolution unless you shrunk the photo with an app etc).