Adobe Illustrator opening all my image files

If I download an image, click to open it, Adobe Illustrator starts up and the image appears there. I have clicked on images, jpg, png, tiff etc. and gone to command+i for information and attempted to change it back to preview many times. I use Mozilla Firefox and have the default setting there to use Preview. I also corrected a problem where the download was dropping the file type from the file name.

I'm a teacher, and this issue is driving me crazy. Any ideas?