Hi, Jess, I've done a few in my own round about way (in lieu of anyone coming to my aid) using my iPhoto and my desktop. It's a "where there's a will there's a way" kind of thing. Putting my paintings/pictures/photos into iPhoto then drag to my desktop then resize them using iResize. I've got iResize 2.3.5 maybe you can find something else but that's what I've been using (there is a free iResize for Mac download) and I keep it on my dock. I'll resize the individual paintings then put them on my desktop and line them up side by side with or without border in between then take a desktop photo (shift + apple + 4) of them together then resize that to the acceptable size for WetCanvas. Here's five of them. The top two are watercolor paintings really small and probably needed to leave the border in between. The flowers are individual photos and the next two are simply one photo cropped to fit. The cookies one is my current signature line. Hope this helps.
WINTER/CHRISTMAS SIGNATURE LINE
WATERCOLORS SIGNATURE BANNER
BLOSSOMS SIGNATURE LINE