Web Graphics General Question - What's The Best Tool???

I have a website that I'm developing using a pretty much standard WYSIWG site builder and for the most part it makes creating a Static page pretty fast with a lot of features.  The question that I have relates to creating roundd frames or boxes, etc. graphics that will be used to group various elements that are displayed on the page.  All of the clip art and built in frames, etc. seem to be square but I'm trying to add a little different flare using rounded frames or boxes.

Say for example I'm using a Blue background for the page, now I want to have a big White box with rounded edges in the center of the page where I can then add various aspects of the page, kind of like the home page of this site.  I can use a text box and change the background color to White but it will always be square.

Would something like PhotoShop have to be used to create that box as a image file then layer that file over the backround and add your other page elements over the image file???

Or, what's the best tool to create such graphics that can be included on a webpage??

Thanks,

ET