Welcome to Stripcreator.
There is currently no way to upload images to the comic rendering engine, but you can convert your comics to jpg or png images and manipulate them to your heart's content offline and then upload or share those images elsewhere (such as our own forums).
To convert to jpg, save the comic, then do a print screen and paste the image into another program like photoshop, paint, etc to play with the image.
To convert to a png, click the Make a Donate button on the bottom of this screen and as soon as Brad has your money, another magic button will appear on your saved comics which will let you convert and save to png in one step. Then you can adjust the png just as you did the jpeg.