Well, since your question is on saving a mat file, theoretically the rest of the code and problem is not relevant and
doc save
would be your first port of call. In your code you are not giving a filename to the save command. Or rather, since you are using the command form of the syntax it is interpreting what you do give as a filename which will not make sense.
I almost never use command form syntax when function form is available. Just something like
save( 'MyFilename.mat', 'imgsdb', 'IMGDB' )
should work fine. Note the quotes around the variable names. This catches a lot of people (me included, even now sometimes!) out. The variables you want to save are given to the save function as strings giving the variable name, not just passing the raw variable.
