hi, i'm currently doing research on cosmic strings and i'm using FITS images to do my analysis. however i would like to add lines into the FITS images that i have so as to simulate these lines as cosmic strings in the images.

i'm not sure if there's any software that i can use and allows me to add lines to the FITS images that i have. hopefully someone can enlighten me on that, thanks!

There are several software Plug-ins for photoshop that allow you do read and save FITS format. Only one I have a link handy for is:
http://astroshed.com/fitsplug/fitsplug.htm
but there are others, Google should turn them up.

I should add FITSLiberator (Google returns a lot of references to it) only reads FITS, doesn't save them in that format so won't fit your purpose. It's the one most of us use as it is very good and free. We rarely need to resave as FITS, (I use TIFF) so it serves image processing needs but if you need the image still in FITS format then only the pay ones, as in the above link, likely will work.

Rick