I just wanted to clarify the way and the settings to convert images to .rat files
as all I seem to get via a mplay conversion is .rat files that will not load and render when used as displacement maps.

R

After some time fiddling with the different settings I found that via Mplay saving your image as a random Access Texture ( TBF Format with Gzip compression works well.For some reason if I also dropped the $F out of the file name I could get the texture to load.

Regarding the loading problem, I'm reading between the lines here, but it is likely an issue with the file browser.
If you turn off “ Show sequences as one entry” you will likely be able to load your image. If a single image has a number at the end and you leave the “sequence” setting on, the image isn't recognized.