COLORIZING: To allow colorizing a clip art, it must fit a couple of requirements. It must be a black PNG image, with a transparent background. For instance:
A good clipart:

A bad one:

Remember to set to YES the colorize paramenter in the settings.

You can add your custom clipart and designs, you only have to deactivate the standard clipart by setting this parameter to YES:

Then, you can upload your own gallery and manage it here:

You can also set the default position of the clipart when it is loaded:

You can even generate a link that preload the design on the product:

About the clipart images, notice that you can set the clipart in 2 places. In the "General Settings" or in the "template's settings page".

If you want to have available the same clipart images in all your product templates, you have to upload them in the "Gen. settings" and in the  "template's settings page" you have to set this parameter to YES.

But, if you want to set a particular set of cliparts in a particular product template, you have to set the parameter to NO and upload the clipart images in the "template's settings page".