Generally you are going to need to bring any images that you download from the internet into a Drawing program like Illustrator (monthly fee) or Inkscape (free) and then either convert the image from a raster image to vector by using the Auto-Trace ability in the drawing program in you want to Score the lines (faster), or if you don’t mind the job taking a really long time, you can probably just let the machine engrave it by leaving it as a raster image. Then you can save the file as an SVG from either of those programs.
If you have never done any digital design before, there are some tutorials that you probably ought to read to help you with understanding the different kinds of images and how the Glowforge treats them.
Starting with this one:
There are other tutorials that will tell you how to save the file as an SVG that the Glowforge can read from each of the programs here:
And if you need any more help getting started, just ask. There are a lot of tutorials for using Illustrator and Inkscape in the Matrix, and several folks around here who know how to use them. 