Possible bug found: Cura 2 install has changed the default application for opening images (jpg, png) to Cura. Noticed on Erin Seaman's workstation (Crane - Debian Jessie running xfce). Initally noticed when opening an email attachment directly from Icedove as opposed to saving the attachment locally.
I see nothing to suggest that. The only thing that the install would do is install the .desktop file (which makes cura appear in the windows manager's applications menus and where it specifies the icon to use, etc..), and the desktop file has a MimeType entry which specifies which files Cura *can* open. images are part of that. It looks like xfce would just select one of the applications that supports opening images and use that to open them. Since Cura becomes 'one of the applications that supports jpg', for some reason gnome decided to use that instead (does it check alphabetically?).
Anyways, the real default application is stored in either /usr/share/applications/mimeapps.list or /usr/share/applications/defaults.list (also ~/.local/share/applications/mimeapps.list or ~/.local/share/aapplications/mimeapps.list, and I also found a /usr/share/applications/gnome-mimeapps.list). Can you check in those files what changed ? They should define the default application to use to open images, and those files are not modified by cura as far as I know.