The showSection param is the identifier in your R script that you used to define your replacement param.

For example, in the following script fragment, you would use 'labkey_png' as the value for your showSection param to only show the image in your webpart:

png(filename="${imgout:labkey_png}")
        plot(c(rep(25,100), 26:75), c(1:100, rep(1, 50)), ylab= "L", xlab="LabKey",
           xlim= c(0, 100), ylim=c(0, 100), main="LabKey in R")
        dev.off()