Have you ever needed to save an image at a certain file size like 122k etc?
You can't easily do that by changing the options of High, Medium, resolution when saving. You can only get close.
If you need to be more precise on the size of the image do the following:
When saving your image go to File > Save for Web and Devices
Select one of the .jpg frames, in this case with 2-Up I selected the bottom image to save as .jpg
Adjust the Quality slider anywhere from 1 to 100 and watch the size of your image in the lower left side of the screen.
When you have the size you need, click Save.
You can specify a specific size for your image.
Again, go to File > Save for Web and Devices
This time click on the little drop-down menu in the upper right hand side of the window and select "Optimize to File Size". (shown below)
Type in the size the file needs to be.
Now you can type in the exact size the file needs to be.