I am using QGIS 2.18 and plugin qgis2web to produce a web map. In QGIS I have the layer layout I want, with a particular image sitting above all others. This layer-order is changed when I attempt to generate the web map (see image). What am I doing wrong? Is there a way to dictate layer order in the web map?
I couldn't recreate this problem if the rasters were not in layer groups. However, when one layer is in a layer group, it seems to be rendered on top regardless of where it should be. However, I see this reflected in the layer order in the Leaflet layers control, whereas your example appears not to be.
As a test, can you try temporarily not using layer groups, and see whether the issue disappears? I suspect you've uncovered a qgis2web bug, related to https://github.com/tomchadwin/qgis2web/issues/175. As such, https://github.com/tomchadwin/qgis2web/issues/ is probably a more appropriate place to discuss.
On the other hand, if you do get the issue even with no layer groups, then I don't yet have an answer.