Outlook keeps asking me how to open jpg files

Step 1:

Open Regedit and navigate to: Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.jpg\OpenWithList

You will have a (Default) key, then multiple keys a-... ,  and finally an MRUList key.

Step 2:

Find the key Microsoft.Windows.Photos_xxx and note the name of key.

Step 3:

Modify the MRUList value and simply enter the key name you noted in step 2.  (In my example it is c but isn't always the case.)