Official English RPG Maker 2003 1.10 release removed the second half picture opacity feature. However, this change is backwards incompatible with older RPG Maker 2003 versions. Both in RPG_RT 1.09a and Player will show the 2nd half as opaque when unset when the 1st half is semitransparent.
The solution for this is just ignore 2nd half opacity for pictures when the detected RPG Maker 2003 is the English 1.10 version. Just as RPG Maker 2000 behaves.
Ignore bottom image transparency in 2k3 1.10 (fixes #539)
Starting with v1.12, the RM2k3 editor will save the transparency value in both parameters (top and what-has-been-bottom) to prevent issues like these in case somebody uses the new RM2k3 to edit old games (even though there will be a few other things breaking, starting with the different RTPs)