Lyn. Are you expecting straight answers
1. Use Photoshop.
2. Use Photoshop.

Aw, OK (make me go and run GIMP)...

No, really. I don't know what "Tab" does, but it doesn't sound like much, so take control...
Pull up your image, do everything you want to it, then, when ready to RE-SIZE, do this...
1. Select Image, then Canvas Size
2. In the box, type in the side you want to make 1200 pixels.
3. Click on the little "Link" symbol just next to the size boxes and this will change the length of the other side.
(NOTE: This will be 800 if you're using a 3:2 ratio.)
4. Click "Resize".
For an illustration to here, see image below.

5. Save the image (Save us from GIMP!) using the EXPORT, then select the jpeg format and give it
a quality setting (Default 99). If you find it's over 400KB, re-do the Export with a slightly lower value till you
get to just below 400KB.

Now, won't you get Photoshop? If you get the CS2 version it's ***FREE**

(And you're wondering why I don;t use GIMP)

Image Below...
USe-Gimp.jpg