I haven't got anything against long topics, it is just awful to hop into one (as I did). You'll normally have to read all replies before you get to know what's going on. However, I didn't, this time.
There are still questions:
Are you using the editor on the mac, but playing the same scenario package on windows?
Because windows has an awful habit to kill the resource fork of every file that has one and is accessable from windows. Thus, Blades of Avernum for windows put the GFX into single files.
Or:
Are you playing it on a mac?
If not, have you exported the pict resources to single bmp files with proper names?
If you have followed every rule, but the game still complains, then I can't help you more.