To save a graph in R Commander:  From the menu bar across the top, select Graphs | Save graph to file | as PDF/Postscript/EPS...  A dialog box pops up.  You can leave all the default settings and click OK.  A dialog box pops up asking where you want to save the graph to, so choose the location to save the file to. 

Next, in webassign, when you get to the graphing in R Commander questions, click the Browse button and find the graph where you saved it.  Select the graph then click the open button in the dialog box.  Remember you have to "submit" your answer in webassign before I'll be able to grade it. 

Hope this helps.  Let me know if you encounter problem.