Unfortunately jpg and png images are not supported as icons (though you can still use them as images). The reason is that raster images can not be easily scaled up and down and so this type of images does not play well with vector nature of WireframeSketcher.
We try to provide an exhaustive set of icons so that custom icons are not generally needed. Along with standard icons provided by WireframeSketcher there are additional icon sets that can be downloaded from our site here:
Let me know if this works for you.
Alex29 Oct 2023
I'm sure custom icons are needed.
because now I personally working on a commercial product design, and all icon sets are private in png or jpg format and therefore I have to copy again and again when using table or other complex components.