Image generators do not handle relative sizes

Some image handlers do not handle percentages well e.g. http://dummyimage.com/460pxx100%/cccccc/000000.png.

To handle this dummyimage could convert relative sizes to fixed sizes based on the other dimension (above url should have been http://dummyimage.com/460pxx460px/cccccc/000000.png) or the default dimensions.