[NTG-context] Why are some images rendered very tiny?

I wrote this macro to display images in my book. It should place the image to the left, filling ~50% of \textwidth, then the caption is to the right. \setupcaption[location=right, minwidth=.5\textwidth] \define[4]\figureis{%     \placefigure{#2, Courtesy #3, photo no. #1\nocite[#4]{}}{\externalfigure[#1][maxheight=.7\paperheight, maxwidth=.5\paperwidth, minwidth=.5\paperwidth]} } #1 = filename, other variables are used for reference/credit details. The problem is, somehow some images are inexplicably displayed very tiny on the page. I check the image file and they are not small, but somehow is rendered small with the code above. What is wrong in my code? --Joel