how can I increase DPI?

I am publishing an academic article where I have some images containing texts and line numbers. I initially created these images by making screen shots, because the line numbering in Word doesn't really copy and paste.

I have now received this message back from the publisher:

Figure 1 has been saved at a low resolution of 156 dpi. Please resupply at 600 dpi.

How do I do this?? :confused::confused: