Yeah I have a feeling it's not the "right" way to use DATA for that kind of stuff, but hey, I'm a noob when it comes to coding =)
Nothing wrong with using Data statements for map data, but it will make your program more expandable if you load the data from disk, rather than hard code it.
I believe AMOS has a "bank" system for saving Binary data to disk which should make the the process quite straight forward.
Do you perhaps have any pointers on writing map editors? Source code would be great, I would actually prefer blitz source, that way I would have to convert them to AMOS.
How you make it is up to you, and more importantly how you organise your graphic data!
I've found that converting source from Blitz to AMOS has been a great way for me to actually learn something. =)
Just have a go at making something like a paint program that let's you drop tiles onto a grid, that will make a good starting point.