You could do this manually with LoadImageFromUrl.
With Webgl this should(In the reallity, never :D) work without problems.
For me it worked fine to make a custom Loading screen that loads the images by steps.
Something like:
LoadImageFromUrl
* 10
On Load finished 10th:
LoadImageFromUrl
* 10
....
This is much work but it works. With some luck anyone else here got a better idea.
Hint: With Ejacta and LoadImageFromUrl you can also load images that are already on the phone/tablet.