Handle image formats not supported in web

[mdbenito]

E.g. pdf or eps images. Either try to convert them (configurable), or in a first approximation just look for alternative formats in the same folder. The config option could be a switch between the three modes: fail, try alternative formats (in sequence), try to convert.

Alternatively add a tag to manually specify alternate formats for images

[mdbenito]

622657c allows manually specifying an alternate image. It is cumbersome but gets the job done. I'm leaving this open because we may want a more automated approach