It looks like IMAQ Overlay Line doesn't let you specify line thickness. You might consider "tricking" it by overlaying two or more lines, displaced slightly from each other but close enough to merge into a slightly thicker line. Note that if you do this, you need to think about the orientation of the line, as you'd want to displace your "sub-lines" at right angles to the original line orientation. For example, if the original line was horizontal, you'd draw two sub-lines a little above and a little below the original line (a vertical displacement), and if the original line were vertical, you'd draw a little to the left and right (a horizontal displacement).
If you decide to go this route, you'll do yourself a BIG favor if you write the "Thick line" function as a sub-VI that looks a lot like IMAQ Overlay Line, but has an additional "thickness" parameter. Hide all of the "messy details" inside this VI, including the calls to IMAQ Overlay Line. This will keep your calling VI "focussed" on the task it is trying to do, without getting "irrelevant" Line-Thickness code popping up and occupying a lot of Block Diagram space.
BS
