I have a jpeg image of a reserve's layout and would like to use it to plot animals movements on i.e. GPS points. I have no idea which coordinate reference system to choose during the "georeferencing" process.
It depends on the region you're working in. As you are using GPS data, i suggest using a UTM system. You can look up a suitable zone on this grid.
As you say you are using an image with GPS data in it the information is going to have been most likely stored in WGS84.
Identify the part of the world you are working in, either by researching the source of the image and or imformation on the image. From there either project the GPS data using WGS84 as the datum into an appropriate UTM Zone that covers your work area.
Or explore what coordinate reference systems are appropriate for the area of interest with appropriate methods of transformation/conversion if nessessary.