I know what you want to do is possible because there is a widget for Zooper on Google Play that does it. The wifi and signal icons are embedded in it so they are hard to use independently, but they prove its possible. Its called camera bar or something like that...
You can eliminate all the other elements from that 4x2 widget and expand just the signal icons.
If that isn't a solution try these two alternatives....
I have mine set up to display the text WIFI, HSPA(or whatever the connection type is for mobile data), or nothing if no data is present.
I used this equation. (wifi is dominant. It shows WIFI if there is wifi and mobile data)
Text and bitmaps are set up different, buy maybe this can help you figure out the bitmap answer. Or just switch to using words instead of icons and the above equation is your answer.
I also used the progress bar feature to create a cell signal bars thing.( Though each bar is the same height.) You could make something like that instead of using an icon and have the color for unused bars be clear so they are invisible.