At the stage of the Design Wizard where you have to define the pieces you have to type the name of the piece, the name of the image, and how you want it to move. You can type anything there, including names with spaces. If you don't type anything in any of those fields, the Diagram will use the default for that item. (Which for the image name is the same as the name, and only well-known piece names have a default move.)
Of course the name you type for the image (or that used by default) should be the name of an existing image file of the correct graphics type, otherwise the piece won't show up in the Diagram when your browser tries to display it. And none of the filenames from the standard piece sets have spaces in their name. So if you specify a name like "asian pawn", you should specify the name of the image you want to use for that (probably 'chinesepawn'). To know which images exist you should look on the page that describes the piece set that you have selected for use in the first step of the Design Wizard. Of course when you are using a piece set that you uploaded yourself, you could have given those images any name you wanted.
This is a bit cumbersome, and this is why I later constructed the Play-Test Applet, which shows you which images are available in the Alfaerie set in the applet itself, often with a name and move specified, so that you can simply drag those to the board instead of first having to define the piece table. It still offers the opportunity to change the name and the move used in the table.
But the Play-Test Applet doesn't offer you the freedom to use other piece sets than Alfaerie, or control the square colors.
At the stage of the Design Wizard where you have to define the pieces you have to type the name of the piece, the name of the image, and how you want it to move. You can type anything there, including names with spaces. If you don't type anything in any of those fields, the Diagram will use the default for that item. (Which for the image name is the same as the name, and only well-known piece names have a default move.)
Of course the name you type for the image (or that used by default) should be the name of an existing image file of the correct graphics type, otherwise the piece won't show up in the Diagram when your browser tries to display it. And none of the filenames from the standard piece sets have spaces in their name. So if you specify a name like "asian pawn", you should specify the name of the image you want to use for that (probably 'chinesepawn'). To know which images exist you should look on the page that describes the piece set that you have selected for use in the first step of the Design Wizard. Of course when you are using a piece set that you uploaded yourself, you could have given those images any name you wanted.
This is a bit cumbersome, and this is why I later constructed the Play-Test Applet, which shows you which images are available in the Alfaerie set in the applet itself, often with a name and move specified, so that you can simply drag those to the board instead of first having to define the piece table. It still offers the opportunity to change the name and the move used in the table.
But the Play-Test Applet doesn't offer you the freedom to use other piece sets than Alfaerie, or control the square colors.