There are two main controls for image file size when saving as a JPEG image.
First is the size in Pixels.
I.e. The number of pixels width x height.
Changing the size in Pixels has a secondary effect of changing the file size.
Eg. an image 3000px x 2000px resized to 1000px x 667px will be a smaller file, 6,000,000px
down to 667,000px.
But is the file small enough?
Secondly, the JPEG compression setting.
File size can also be adjusted by setting the JPEG quality (1-100) when you save the image.
The quality is also another way of saying which compression level you want.
The more compressed the image the more detail you may lose.
So to size an image for AP you should set the pixel size you want (typically no more than 1024 on the longest edge)
and adjust the JPEG quality (compression) to get the file size under the 250kB limit.
Some other advanced controls that have a minor effect on the image size are:
- Including meta data all, in part or none
- Saving baseline or progressive (and how many scans, 3 or 5 etc)
- Saving colour profile information (don't bother with sRGB as that's the web default)
Note: You should always make sure the image is saved as an sRGB colour profile for web publishing.