The black outline of the controller is too light to see properly. I don't have Photoshop or any paid photo editor. I'm not a graphic designer and this is a one time project for my job at a small business. We only have 2 employees and neither of us are graphic designers, so it would be great if someone could tell me how to do this without any expensive software. I've also tried Photoshop in the past and it's too complicated for me.
Nearly all photo editing software have the curves tool. It's handy for adding contrast manageably. You can download a freebie such as Krita, GIMP(difficult) or Paint.NET(easy, but their website pushes adware) or take a trial of commercial software. This is in Affinity Photo:
Check if one of your friends who adjust photos can do the job.
Your included image in the question has very low resolution. The image is originally drawn as a sharp vector drawing and it has been rasterized (=converted to JPG or PNG) with too low resolution. No idea was it you or the source of your image. Rasterized line drawing is grey if the original black lines were too narrow to be shown in the used pixel resolution. The same happens easily when high quality prints are scanned with low resolution. As well the image can originally have grey lines - that obviously has happened in your case.
Your case is special: the image is available as vector - it's in Futaba's manual:
Download it as PDF, open in Inkscape(free) and do the needed edits. I changed black lines and fills to blue and grey lines to black:
I selected Poppler-Cairo PDF import mode, opened only one page and before editing I selected all and started Extensions > Arrange > Deep Ungroup to make all elements separate. All grey lines can be selected by selecting one and then launching Edit > Select > Same stroke color.
From Inkscape you can export the image in as big size and in as high resolution as wanted. All texts can be changed and line widths are also editable: