You can initialize a layer by giving an uploaded image file: it consists to initialize a layer from the temporary path of the file.

Considering that the file input of your form has a name attribute called "image":

$layer = ImageWorkshop::initFromPath($_FILES['image']['tmp_name']);

If you are using a framework, this is similar: find how to get the tmp_name of your uploaded file, and if you can't, first save the file and then open it as a layer from its saved path.

Be sure to give an image file.