Need to dynamically swap .jpgs in a matrix of button fields

Created an X/Y graph with a matrix of hidden buttons, all assigned the same .jpg. When a condition is met, a button field becomes visible and creates a plot point on the chart. When the form is used a second time (re-test), I want to swap the .jpg (for all the button fields) with another .jpg to create two series of plots (of different colors) to depict two separate measurement periods.

Just as I manipulate these button fields from hidden to visible (and back),

if (stdscore_i == 8) {
this.getField("btnI_8").display=display.visible;
}

I want to do the same with the .jpgs.

I would greatly appreciate any suggestions.

rhunter