Hi Meego!
To import an image into your project first place it in the same folder where you keep the other images that are used by the skeleton. You will see it come up in the Images
folder in your Tree
view. You can simply drag and drop it in the viewport to have it in your skeleton.
Here's a gif:

Regarding your second question, the feature to crop images is planned, but for now you'll have to use an external program like Photoshop, Premiere, After Effects, or any other image/video editing software to resize it.