If you want Gimp to simply open image files each time, eg jpgs, then cmd click ( right click) a file and select Get info.
In the window, go to Open With, and select Gimp and then hit change all.
That will then change all jpgs to open in Gimp by default. You will need to do this for each type of image file if you want to them all by default.
Gimp may have a selection for this in its Preferences, but I haven't used Gimp in a long tome, so i'm not sure ther.
So if you CMD click (or Right Click) on the jpg, you get the contextual menu.
There you can select Open With .... and select Gimp from the list.
However, if you want that to stick and always happen, sect Get Info instead and in that Get info Window, Select Open With and change the Application to Gimp. This will change it for just that file.
However, if you also hit Change All there, it will change for All jpgs on your computer
I'm at the end of my rope. I just want a good simple graphics editor for the Mac, like PaintDotNet for PCs. Everyone says Gimp is the ticket, but I can't get it to open on my daughter's old macbook, Leopard OS X 10.5.8 (9L30.) I've downloaded and installed Gimp 2.6.11. I have X11- XQuartz 2.1.6 (xorg-server 1.4.2-apple33) - running. From all I've read on Gimp/X11 support groups, this is the correct combination.
But every time I click the Gimp icon, allI get is this error message:
The application Gimp quite unexpectedly.
Mac OSX and other applications are not affected.
Click Relaunch to launch the application again. Click
Report to see more details or send a report to Apple.
Contents of Report:
Process: Gimp 
Version: ??? (???)
Code Type: X86-64 (Native)
Parent Process: launchd 
Interval Since Last Report: 34382 sec
Crashes Since Last Report: 9
Per-App Interval Since Last Report: 0 sec
Per-App Crashes Since Last Report: 9
Date/Time: 2013-06-11 17:28:36.581 -0400
OS Version: Mac OS X 10.5.8 (9L30)
Report Version: 6
Anonymous UUID: D8E65660-07B1-4410-A2AA-CA3DCC3D7E03
Exception Type: EXC_BREAKPOINT (SIGTRAP)
Exception Codes: 0x0000000000000002, 0x0000000000000000
Crashed Thread: 0
Dyld Error Message:
unknown required load command 0x80000022
PS. Normally I click the Gimp icon with X11 closed, expecting Gimp to open X11. Once, in desperation, I tried opening X11 first, but the same thing happens.
Thanks for reply, but I have succsessfuly downloaded gimp 2.8.2 with all the extra scm addons and it is great, but-- I am unable to download two items which I really want and that is Gmic and the full Beautify scms so I have tried 3 times to download separately from Simone Karin Lehmann who says it is compatable with X Lion 10.75 ---It crashes every time, I have installed the latest XQuartz and tried setting a direct path to folder , but nothing, I am not keen on uninstalling the Gimp 1.8.2 incase I cant get the 2.8.6p1 working.Is there any thing else I can do?