Firstly, i should say I'm an elementary os newbie.So I'm really sorry if my question is too basic, pretty please please be patient with me, I'd really appreciate your help. I installed Elementary OS Freya 2 weeks ago, my first Linux based operating system. I had the usual pictures icon down on my desktop, but i was "smart" enough to try to install a program called "gnome wallpaper slideshow". I got a warning that it gets into conflict with an already existing program or something like that (i think with the embedded photo viewer) but i ignored that warning and installed the program anyway. Now, when I try to open a .jpg (right click-open with...) i only get the option "create background slideshow, and when i click on it nothing happens. I'd like to uninstall this program but i can't seem to find it anywhere. I'd really appreciate your help. I would like to be able and see .jpg files just like it should usually work in Elementary OS. Thank you for your help!
If you want a simple uninstall
$~sudo apt-get remove 'application_name'
Or you can use
$~sudo apt-get purge 'application_name'
to uninstall the application and all relative files.
It seems in your case like you changed the default application that reads .jpg files.
Go to > System Settings > Applications > Image Viewer
And choose the default elementaryOS image viewer