• I use Photoshop.

• I need the file size to be AT MOST 40kb

Answer:
The easiest way to re-size a pic to an (almost) exact file size is to use the 'Save for Web' option.

To get the menu:

Go to file,

Click the ninth option on the list.

Or use the keyboard shortcut, Atl Shift Ctrl S

To use this:

In the wizard that appears, select from the right-hand drop-down box 'JPEG'.

Then adjust the quality of the jpeg. Using either the slider or the preset menu (Low, medium etc.).

Below your image there is some data that tells you how huge the image will be when saved. (It is shown in K).

Click save.

Answer:
or go to image –> image size –> either change the resolution or the image size! easy… then make sure you save it as a jpeg, it's the smallest file format for images.
Answer:
File / Save for web and devices.

It make the filesizes tiny with a negligable reduction in quality.

It's the way forward.

Answer:
1. Change its resolution or its size,

2. Crop unnecessary..

.PS.

.if you wanna send it, then zip it..

Answer:
to reduce an image go to the Image> Image Size menu
Answer:
save as.>.jpg>Quality>Low

simple

Answer:
use .png format instead of .jpeg