No, you can reload them from storage. Keeping two copies of every drawing resource decompressed and in memory all the time in case of context loss would be very inefficient
That's a nice theory which falls somewhat flat. LocalStorage, the offline file cache and the client Web SQL database are limited to 5mb of data tops (some platforms obviously assign a lower hard limit). IndexDB doesn't work on any mobile device yet. So the only way to "reload from storage" for any remotely serious usecase would be re-fetching from the server. So a context lost can easily results in a serious amount of "re-boot" time. And then there's obvious issues like that some resources (like images) may be difficult to get array buffers from on mobile devices, so putting them in client storage would be unrealistic anyway. In theory you could keep the Image instance around, but in practise that amounts to pretty much the same thing as just holding the bytes in working memory.
So if you have a choice between half a minute "re-boot" from server in case of context lost, or holding in ram and be back in a second or so, I know which is gonna be used by just about everybody after the first time trying the alternatives.