While using MapQuest’s Javascript API with OpenStreetMap to create a mashup that displays contractors on an interactive map the map suddenly stopped rendering tile images. Instead the MapQuestion cogs background was shown.
The Solution
After doing some research we discovered Twitter Bootstrap contained CSS that makes every image tag have a maximum width of 100%.
Removing this CSS rule for images within the MapQuest map containing div resolved the problem.
#map img { max-width: none }