If the Quicktime Picture Viewer application is defined on your computer, also called associated to, JPG files, then that is the one application that will be defaulted to open your JPG files.
You can change that association. Usually, when you install some applications that can be used to work with some files, you can opt to use that application to open those types of files.
For example, if you install some media player, then it may default to open all sorts of media files (WAV, MP3, etc.). If that is not what you want to happen then you have to ensure that you deselect that option.
But, if you missed that point during the install of your QuickTime application, you can still change it more to what you want.
Just click on a JPG file, do a right-mouse click and see if there is an [Open with] command. If so, then select that and see what other applications you can select from to use. Your desired application may not appear, and if not then you have to use the selection to [Choose default program] command to drill down to the program you want to use.
Note: if you are sure that is the program you want to use now and every time from now, make sure the "Always use the selected program ..." is checked. otherwise the choice you make now will not be remembered for the next JPG you try to open.