Welcome to EpoS Wizard!
So I finally figured out how to get this to work.
I am not sure if there are different versions of the CV-33 software but I don't have anywhere as many options Communications tab for files to upload as those listed by deisetills.
What I did find was that if I add the bitmap to a key and check the 'Transfer bitmap file' and then, with the keyboard layout still in view, I click on the 'Send File to ECR' icon on the ribbon bar. This seems to update all the files required, without being given an option to select, and after a significant time (2-3 minutes) the new bitmap appears on the ECR keyboard.
Its not CV-33 that dictates the options you see in the communications tab, its the IPL version on your particular till. CV-33 is quite old software (20+ years) and if not run on just the right system (xp 32 bit only) it can be a bit slow/buggy in places. But you got a result and thats good 
