No apologies necessary! Sorry if I was a little strident, but this comes up over and over again, and it seemed like the time was ripe for a public-service announcement. ;-) (And while bandwidth is far less of an issue these days than it was 15 years ago, I'm still an efficiency geek. Why use 3 KB when 1 KB would suffice?)
Anyway, I'm most familiar with gnuplot, which has both "png" and "gif" terminal types; a simple set terminal png or set terminal gif command will select one of them, and you can set the filename with set out 'foo.png', for example. Then replot or whatever you normally do.
xfig is a vector editor that uses its own .fig format by default, but it can export to all sorts of things, including PNG and GIF, if you've got the requisite helper apps installed. I think I've used it about once--and more than a decade ago at that--but according to the manual, you just bring up the export menu and select "Language" (a.k.a. output format) from the dropdown at the top. Alternatively, you can set the Fig.exportLanguage resource in your .Xdefaults file. Or you could just save to PostScript and use Ghostscript to convert to PNG or GIF, or ps2pdf and your favorite PDF viewer, or use a screen-capture utility.
I haven't used GNOME's screenshot function and don't actually have a GNOME desktop handy anymore (Slackware dropped it :-) ), but I'd bet it's based on imlib2, which has had PNG support pretty much forever. Alternatively, if you've got PBMPLUS/NetPBM installed, you can do xwd -out foo.xwd (and click on relevant window) and then xwdtopnm foo.xwd | pnmtopng > foo.png (or similar). No doubt ImageMagick/GraphicsMagick are also capable of doing such conversions. And then there's XV, which I currently sort of maintain (for loose definitions of "maintain"); it's not Free software, but it's really well written and can capture screenshots and write them out in many formats.
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