I found out that some images were missing from the final exe files, but were correctly shown as "catched" in the source code.
I could not understand why until I realized that even if the code is correctly updated when using Image Manager, the form file is not considered modified (i.e., no * after the .pbf filename), so the updated code is not really used when the main file is compiled.
In code view, the * comes and goes quickly, in design preview I never see it appearing.
Instead, if any gadget is modified then the * appears correctly.
