After launching ImagePacker you'll see this layout:

The main canvas area displays the packed images. In this window you can click on any of the images. Clicking an image selects it (as inidcated by a highlight box).
The image list to the right shows what images are available
for packing.
Each listed image has a symbol to the left its name.
The status of the symbol changes to indicate the following:
Green tick - Image is currently packed
Red Cross - There is no room in the pack to store the image
You have several options to assist you in putting a pack together
Loading images
a) Menu > Load Pack - Loads a previously saved pack of
images
b) Click Add button to add an individual image
to the pack
c) Click Add (Folder) button to add a set of
images from a folder
d) Click Remove button to remove the currently
selected image from the list
e) Click Rename button to change the name of the
image
Image manipulation
There are a variety of image
manipulation buttons available.
Saving the pack
Once you have your pack assembled you can save it as one single larger image.
The pack is saved along with a definitions file
which details the images stored in the pack.
You can also generate source code
for testing your packed images/sprites with.
Help files
Some options in Image Packer have a [?] button. Click this to open the associated help guide.