If instead of setting the graphics fixed in your program you prefer that your end users are able to do it, you may activate the "User Variable" choice in the "Graphics:" option.
If that is the case, you must specify the name of the variables where the name of each graphic would be stored. As in this case the file is variable, you may not include it in your application project, thus it must be read from disk for each button. Depending upon user's machine, this might have performance issues. Besides, you must distribute the graphics with your program, otherwise they will not be picked in your user's machine and the program will look odd. You may avoid that, including every possible graphic in your project, and when the user select a graphic adding a ~ character before the path+name if it's in your list of graphics. You must keep in mind that your executable will grow as much as the sum of all the graphics included.
The variables specified to include the graphic's name should be big enough to store their name with full path if necessary. Normally STRING(256) will do. The variable name drop-lists will shown only the global variables that contains "button" or "boton" as part of the name (for example, "GLO:BigButton").